Air spray gun for furniture finishing

Air spray gun or air conventional spray gun is a type of spray gun which was first discovered and used in the finishing materials application. It is the initial basis of the spraying equipment before the other type were developed. In this spray gun, the finishing materials is flowed together with compressed air through a room called nozzle. Nozzle is the spray gun part that is designed to let the mix material flows in very high speed. The quick flow of the finishing material and the air flow make the atomization occurs. And then the atomized finishing material can be directed by the gun to coat the surface needed. 
Atomization is the condition when a liquid becomes a diffused of small particles material. In the spraying process, the atomized material then is directed and layered to the wood surface. The atomization is producing an excellent layering result. The air spray gun gives the best atomization of the finishing material and will produce the best result in layering materials. This equipment give the smooth, uniform surface and most flexible in use with fully adjustment. The air usually spray work at 4 bar (14.7 psi) air pressure, but it also depends on the viscosity material that was applied. The higher viscosity of the material require the higher air pressure.

The advantage of this air spray gun are: very flexible in use, produces excellent atomization, produce very smooth and uniform film, equipment is relatively simple, inexpensive and has high flexibility to be adjusted according to the application requirements. Air spray gun can be used for almost all kind of finishing materials applications. The air spray gun is very flexible and fully adjustable. The operator can adjust the air pressure, the volume of material, the spread width of the material and also the direct of the spread of material. This equipment has high flexibility to be adjusted to handle the variation in the production process. It is more suitable to do the production with varies products with the small to medium volume production.
Meanwhile, the biggest disadvantages of the air spray gun is his low transfer efficiency material. The transfer efficiency is range from 20% to 30% depend to the product shape and the operator skill. It means that only about 30% of finishing material will layer to the surface, and the rest (about 70%) are spread as waste in the air. The quality of the finishing resulted is highly depend to the application technique. To get the good result ( in the finish result and the efficiency), the equipment always requires a good skilled operator. In the big volume job when the long man hour is needed, it may lead to the inconsistency result  since of  the operator’s fatigue.

The atomization finishing material in the air spray gun

H.V.L.P. spray gun

To increase the efficiency of transferring material at the air spray, we need to keep the air pressure low. The air pressure used in application using air spray gun should be set in the position at the lowest pressure but is still enough to do the atomization of finishing material perfectly. The higher pressure of the air pressure will lower the efficiency of transfer materials.
To improve the efficiency of material, the air spray gun was developed and modified to the  H.V.L.P. (high volume low pressure) spray gun. This spray gun has the same basic system with the air spray gun. The H.V.L.P. spray gun is designed to maintain the air pressure in the spray gun is remains at the low pressure (about 0.7 bars). Then to improve the quality atomization material the more volume air flow is used. The using low air pressure will  minimize the dust spray and increase the efficiency of the finishing material transfer.
The disadvantages of this equipment is the low speed and the work for the limited viscosity material can be handled. This equipment cannot used for materials with high viscosity such as: sealer or top coat. This equipment usually only be used to spray the stain and glaze. The other disadvantage of the H.V.L.P. spray gun is that he needs more volume of compressed air than the air spray gun.

UV Coating for wood finishing

Ultra violet cured coating (UV coating) is the other answer for the requirement of paint product which is more environmental friendly. The UV coating is very less in releasing the "expensive and pollutant" solvents. It also require little energy for drying and produce a very strong film layer. The ultra violet cured coating drying very fast, he need only few minute to be dry and produce hard coating. The curing reaction stimulated and catalyzed with ultraviolet radiation. The reaction mechanism is quite complex, but it can be simply described as follow: 
The coating contains a polymerized resin that will do the polymerization reaction with an energy ignition. When the coating is irradiated with the ultraviolet light with certain intensity, then free radicals that will accelerate polymerization reaction are generated. The polymerization is curing the coating to make a hard and strong film layer. 

 Roller coater for UV coating application

There are 3 components required for the formation of film layers, polymerized resin, photo initiator and ultraviolet light. The resin is the material that will polymerize to create a layer of film. Photo initiator is a substance that absorbs radiation from the environment into the resin to initiate polymerization reactions, with photo initiator spurred by free radicals is so fast and then formed to begin the drying reaction. The ultra violet light is the energy that a needed to initiate the reaction. 
The resin material that used to make UV coating is usually oil free unsaturated polyester or acrylic. And the initiator is benzoin used for polyester and acetophenones for the acrylic. The UV coating will produce the hard and strong film coating in the finishing.

The advantages of UV coating

• The resulting film layer is very strong and hard, resistant to scratches and solvent.

The film produced by this coating is very strong, stronger than the the acid curing or polyurethane clear coating. The UV coating is the proper material to be used for the finish that require the strong and high resistant film.  It is most used to finish for he parquet flooring or top tables that need maximum protection and strength.

• The drying process is very fast, and does not require high temperatures

With the help of UV ray,  the coating will be dry very quickly. The application process is usually  done in a series of equipments; a roller or automatic spray gun to apply the coating and the UV light to cure the coating. Out of the equipment the coating is already totally dry and hard film is already built. The product is ready to be packed; it does not need long drying time. No need the drying room and the finishing process is done very quickly.

•  End at relatively low cost compare with another coating.

UV coating material is made with a high solid content, up to 98%. This material almost does not use thinner either in the application or in the mixing. All the paint materials that is applied  will coat and form a film layer on the surface finishing. This material is more efficient in application; although the price of coating is relatively more expensive compared with others, but it need less amount materials to make the same thickness of film layer. 
With less solvent releasing, this coating is also very friendly to the environment.

While disadvantage of this coating are:

• This coating requires a series of machines an automatic spray and ultra-violet lamp that is expensive.

This paint will dry only if there is an ultraviolet light with sufficient intensity, so the application requires a series of equipment with an expensive ultra-violet lamp as oven. This paint is usually applied in a set of equipment the first is a roller coater or automatic spray equipment, then followed by UV lamps for drying. The UV applicator equipment is considered as big machine which is expensive.

•  Its uses is limited to a flat product.

This coating needs UV lamps for drying, whereas the UV ray is harmful to health. The UV ray can cause cancer if expose to human skin. Since that, the UV lamp circuit should be in a closed circuit. This condition limit the uses of the UV coating. Until now the coating is only workable to be applied to the flat panel. It may still can be applied on the uneven surface, but still on the flat form. Althugh theologically it can applied in complex form by spray and dried in special room with UV lamp, but it will create difficulty in the sanding and it does not economical. 

Water-based coating for furniture finishing

Waterbased coating could be the answer from the finishing material industry to provide the material that is environmental friendly. Water based coating is the finishing material that is purposed to provide the environmentally friendly and safe for the finishing process. Water-based coating uses water as its major thinner. It releases water at the application and the drying process which is more safe and environmentally friendly than any solvent. The modern finishing process will always "give" pollutant to the air and environment. All the solvent that used in finishing process will evaporate in a fume to contaminate the air and environment. The gas fuming is released during the application and drying of the finishing materials. The spray process which is very popular in the finishing industry always produces dust sprays that leave to the air and the environment. 

Now when the peoples more consider about the safe and healthy environment, the finishing process especially in the developed countries must be done with a very strict rule according to the cleaning and environment aspect. Thinner and finishing materials are also consider as very flammable materials, so finishing process also requires a tight safety rules to prevent fires. The gas released is restricted in many country, and it attract people to use the water-based coating.

Water-based coating usually is made of acrylic resin, modified acrylic or “one component polyurethane”. These materials actually are not soluble in water, so the resin mixture is dissolved in a solvent that evaporates slower than water. In the drying process, the water as the major thinner is evaporating before the solvent. After most of its water evaporated then the paint solution will release his solvent to make a dry film layer.

Into the water-based coating is added a surfactant to make the mixture disperse into the water and form a homogeneous emulsion. Surfactant used is a kind of soap and often creates bubbles during application. Defoamer is added to reduce the bubbles was. To the mixture of materials are also added some other additives to obtain specific properties desired. The resulting film layer is termosetting layer that can not be dissolved again by the water, or solvent. This film layer is relatively stronger compared with nitrocellulose coating, but still less strong then  the acid curing or polyurethane coating.

The advantages of water-base coating

• Non-flammable and produces less air pollution.

Water-based coating actually does not mean not using a solvent at all, Water-based coating also uses solvent, but in a small amount because his major thinner is water. Since that the finishing process with water-based coating is much more secure than the solvent base coating. Water-based finishing is also more environmentally friendly than solvent base paint. Now when the environmental is a very important issue, then the uses of water-based coating is very interesting.

•   The resulting film coating is strong

The film produced by water-based coating is a termosetting film that will resistant to solvent and scratches. This film layer is stronger than the coating film produced by nitrocellulose coating, but less strength than the acid curing or polyurethane clear coating.

The disadvantages of water-based coating

• The resulting appearance is slightly cloudy. 

The drying of this coating is with the chemical reaction. The reaction will affect the clarity of the final film result. For high end furniture products that need the beauty and clarity of his finishing look, this coating cannot produce the look as the solvent base material.

• The application is relatively difficult

The paints use water as its main thinner, while the water is more difficult to evaporate compare with solvent. Therefore, the water-based coating needs longer time to dry, compared to solvent base coating. This coating is also more sensitive to environmental conditions, especially the temperature and humidity of the air. When the environment air is dry and hot, this coating can dry quickly, but at a cold temperature and high humidity air, the paint will be difficult to release his water into the air. Therefore an oven is recommended facility in the finishing room for water based coating. The oven could help to the finishing process by control the air temperature and humidity. Another problem in the handling of water based coating is the equipment and tools that used to process and storage this material. Water is more corrosive material compared with solvent, so the tanks and equipment used for storage and applications should made of plastic or stainless steel which is more resistant to water corrosion.

Polyurethane clear coating for furniture finishing

Polyurethane coating is the coating made from the urethane polymerization reaction. The urethane is the result from the reaction between the poly isocyanate with resins that contains the hydroxyl group such as: alkyd, castor oil, urea, acrylic, cellulose, polyester, etc. The common resin to make PU coating are: the polyester and saturated polyester. Isocyanate is a toxic substance and usually is used as prepolymer with pylol to minimize his gas emissions to the air. According to the resin used, there are many types polyurethane coating. But the polyurethane coating that is most widely used in the furniture finishing is the 2 components polyurethane. It is the polyurethane coating that is made from the hydroxyl group resin and the isocyanate. In the application both chemical is packed separately. The resin is mentioned as the coating, while the isocynate is the hardener. In the application, both material need to be mixed in the proper composition to make the Polyurethane coating.
The application and handling of the polyurethane 2 components must be done carefully; the thinner used should not react with the isocyanate. Materials such as: water, alcohol, and glycol should be kept out of polyurethane. The isocyanae should not be exposed at the open air; the water in the air could react with the isocyanate and make the make it dry and harden. 
The polyurethane coating for wood furniture is used, when the furniture need strong film and maximum protection. The coating resulted is high build and strong, resist to the scratch, chemical, solvent and chemical. 

The advantages of polyurethane coating

• The resulting film layer is very strong.

The film produced by polyurethane coating has high strength, resistance to heat, solvents and scratches. The polyurethane clear coating is believed to have greater hardness and strength than the acid curing. The coating is used for products that require strength and maximum protection. It is the common coating for the: cafés, bars, hotels, furniture. It also can be used for the dining table, kitchen cabinet and some wood flooring. 
The PU coating is also the proper material to make the high gloss and high build film finish. With its high built and strong film layer, it will give you the perfect high gloss finish.

• The resulting coating film has a soft appearance.

This clear coating will give the soft appearance of the film layer is very unique and liked by many peoples. The look of this finish will be difficult to duplicate with other types of paint. For people who is familiar with the the polyurethane appearance of course will have more receptive to the finishing product with this coating. The Polyurethane coating is most used for Italian furniture. With its high built film and its soft and special looked it bring the luxury to the Italian style furniture. 

• Polyurethane coating is considered less toxic than the acid curing.

Some people think that the polyurethane is more healthier and environmental friendly than the acid curing coating. This is actually not exactly right, because both isocyanate and formaldehyde are substances that are toxic and will interfere the human health. In fact gas emissions depends on the quality of the coating itself. 

• Polyurethane has high solid content.

The polyurethane clear coating has a solid content higher than the acid curing or nitrocellulose at the same viscosity. Therefore this paint will be easier to produce finishing with thick film. The film is thermosetting, then he has ability to fill and close the pores and grain with many layer application.

The disadvantages of Polyurethane coating

• The price is relatively expensive.

The PU usually is more expensive compared with the acid curing or nitrocellulose paint. The coating is also has shorter life time then the NC or AC. The isocyanate is easy to react with the water in the air; it should be kept in the closed cane and put in the dry air room for his longer life. The proper handling and storage system is needed, and it will increase the price of the chemical.

• The application is difficult.

The polyurethane is very sensitive to the environmental conditions, the material consists of 2 components with short pot life. The PU coating will hard after 1 hour after being mixed with  its hardener. The perfect mixing ratio is needed to get the good result.
The polyurethane is very sensitive to the humidity environment. In the humid and cold air it easy to get problem with the blushing. He as long time to reach the touch drying. It means that the coating is sensitive to the dust. A little dust that does not cause problems in the nitrocellulose coating, will be big problem for the polyurethane coating.
The finishing room and the drying room for the PU coating has to be clean and dry; free from dust and humid air. 

Precatalyzed clear coating

The precatalyzed clear coating is the Acid curing clear coating which is made in 1 pack component. The other disadvantage of acid curing clear coating is that the material consists of 2 components of the paint and catalyst (hardener) which must be mixed with a certain ratio when used. After the paint is mixed with his hardener it will react and dry or harden, although still mixed with his thinner. The time for drying after the paint is mixed with his hardener is called pot life. This make the paint is more difficult to be handled than nitrocellulose paint which is dry without chemical reaction.

Therefore to make easier in application, the acid curing paint has been developed to the acid curing with 1 component, known as acid curing precatalyzed. Precatalized acid curing clear coating, as precat sealer or top coat is an acid curing sealer or top coat but it is a 1 component. The paint and hardener is mixed at the time when the paint was made; the chemical reaction will occur during the thinner evaporation. This paint is easier in application, but the resulting film layer is not as strong as acid curing paint with 2 components. The resulting film layer is also thinner than the 2 component acid curing paint.

The 1 pack component make the application is easier, but it also shorten the life time for the coating. The mixing of the hardener and the coating makes the material will spoil faster then the 2 component material that are packed separately. The melamine coating can stay for 1 year in the good condition when the coating and the hardener are kept separately, but the precat coating usually only has 6 months life time.

Acid curing clear coating for furniture finishing

Acid curing, melamic or melamine clear coating is a coating that uses an amino resin in the mixture. The amino resins are the product of the reaction between the formaldehyde with materials containing amino group. The amino that most widely used is a melamine or urea, that way it is also known as melamine paint. The amino resins when it is used alone will produce a hard and brittle film. In the coating it is used in the combination with other resins such as: acrylic, alkyd or polyester. The amino resin will do as a hardener (curing agent) or the bond-forming polymer chains (cross linking agent) to make a strong, insoluble and thermoset film. The film result is hard, high resistant to scratches, solvents and heat.

The advantages of acid curing clear coating

• The film produced is hard and strong. 

The resulting film layer is formed from a chemical reaction that is strong, resistant to heat, scratch and solvent. It is suitable for finishing that needs stronger film resistance such as: dining table, kitchen cabinets, desks, furniture for hotel, bar or cafe, etc.

•   It has higher solid content. 

This coating has higher solid content than nitrocellulose with the same viscosity. This coating can produce a thick layer film with less application. When it is dry, the resulting film layer would not be resolved with solvent. It can be used to make close pores finish faster and easier then the NC finish.

The disadvantages of the acid curing clear coating

• Produce formaldehyde gas emissions.

The uses of amino in the formation reaction, make the paint will emit formaldehyde gas during its curing process. Formaldehyde is hazardous gas; it could interfere human health if it is inhaled. The curing reaction to produce dry film actually is relatively fast about 24 hours. But the reaction is still continue running for long time so that the gas emissions still is occurred for a long time even with very small amount. It makes the finish still smell formaldehyde for a long time. 

• The film result is thick, tend to look like plastic. 

The resulting film coating is the result of a chemical reaction make it has slightly blurred appearance. The film has less clarity compared with the nitrocellulose clear coating. Too thick  application of the avid curing coating will make the film is thick, less clarity and look like plastic.

Cellulose acetate butyrate

The butyrate or the cellulose acetate butyrate (CAB) lacquer is the non yellowing lacquer. It is a special lacquer that is developed to answer the problem of yellowing in the nitrocellulose system finish. It is formulated by using the cellulose acetate butyrate resin as the main ingredient. Cellulose acetate butyrate resin is cellulose (wood fibers)  that is processed by acetic acid and butyric acid. The cellulose acetate butyrate coating has high resistant to the UV light. It is simple, one component and quite easy in application as the nc coating, but its film won,t be yellowing.
The cellulose acetate butyrate resin is mixed with acrylic resin and some additive to make butyrate lacquer. The butyrate clear coating will give a film layer that has a better resistance to the ultraviolet light. As a lacquer the butyrate is quite similar with the nitrocellulose lacquer. It is one component coating, washable, fast dry, clear looked and easy to be handled.

Compare with Nitrocellulose coating, the cellulose acetate butyrate coating has less resistance to the alcohols and hydrocarbons solvents. The cellulose acetate butyrate film also has less compatibility with modifying resins and plasticizers. The butyrate lacquer also will give the "muddy looked") to the finishing film. The muddy looked is very slightly and will not affect the white or light color finish, but it could be visible for the dark brown or black finish. 
Normally the cellulose acetate butyrate is only used to the light color or white color when the yellowing film is not allowed. It is more complicated and more expensive then the nitrocellulose lacquer. The butyrate lacquer is the proper coating for the white color finished such as: white paint, white wash or antique white finish. It is also used for the natural for the light wood finish when the yellowing of the film coating is not allowed. 

The butyrate clear coating is available in the sealer and top coat. The butyrate sealer is rarely used. The addition of the sanding agent to the butyrate is limited. The small amount of the sanding agent will quickly increase the viscosity of the lacquer. In the finishing process the butyrate is used for the top coat and sealer for the white color and very light color finish. The combination with vinyl sealer is also can be used. The vinyl sealer is not yellowing sealer and will increase the coating adhesion. 
The butyrate lacquer mixed with white pigment is the white enamel. It will give the white paint color and can be used as the last coat layer.

white finish with butyrate lacquer

Spraying equipment types

As well as furniture finishing industry, the finishing equipment industry is also growing rapidly. Various types of finishing equipment such as: spray gun manual or automatic, electrostatic spraying equipment, roller coater, curtain coater, etc. has been created to support the finishing industry. It is synergism with the finishing materials industry that are also produce the special finishing material in accordance with many finishing application method such as: dipping, spraying, flow coating, powder coating, etc. The finishing process now can be done much faster with better results.
One of the equipment for application finishing materials that is widely used in the furniture finishing is the spray gun. Now most of the finishing work is done using spray gun, either manual or automatic spray gun. Spray gun is equipment for the application finishing materials that available to atomized the finishing material and then direct it to coat the desired surface. By using the spray gun, the finishing material can be applied to the surface quickly and easily with high flexibility in the adjustment to get the desired thickness. The advantages of the spray gun is he can provide the smooth and uniform finishing layer, and its flexibility in application. The other coating application machine such as: roller coater, curtain coater, disc spray or other automatic machine, but they will not as flexible as the spray gun.

Types of spray gun

There are many types of spray gun available, ranging the smallest and simplest way cup gun used manually to small scale production up to the automatic spray equipment used for the big volume production. However base on the way of atomization of finishing material there are only 2 types of spray gun.  That are: the air spray gun and the airless spray gun. Both types of spray gun were developed and modified into the some more other types such as: air assisted airless spray gun and h.v.l.p. (high volume low pressure) spray gun and l.v.l.p. (low volume low pressure) spray gun.

The types of spray gun base to the way of atomization of his material

• Air spray gun: the type of spray gun which is using air to make the atomization

• H.V.L.P. the spray gun which use low pressure air and the high volume air to the atomization

• Airless spray gun: the type of spray gun with use high pressure and without air to to the atomization

• Air assisted spray gun: the airless spray gun with the air flow to improve the atomization
  

Spray gun	Advantages	Disadvantages
Air spray	- Low price	- Low efficiency transfer material (20%-30%)
	- High flexibility	- Need skillful operator
	- Excellent atomization
	- Suitable for most kind of materials
	- Easy to be adjusted
HVLP	- Low price	- Low speed
	- High efficiency material (30%-60%)	- Need more air pressure
	- Easy to be adjusted	- Cannot use for high viscosity material
Airless	- High efficiency transfer material (30%-60%)	- High price
	- Work for high viscosity material	- Atomization is not as good as air spray
	- High speed	- High price
		- Less flexible
Air assisted airless	- High speed	- High price
	- High efficiency transfer material (40%-60%)	- Less flexible
	- Atomization is better than airless	-Less control to the equipment
	- Work for high viscosity material

Nitrocellulose coating

Nitrocellulose is coating that uses nitrocellulose resin as the main ingredient. Nitrocellulose resin is made from cellulose (wood fibers) that was cooked and processed with nitric acid and sulfuric acid. Pure nitrocellulose resin (when it is dry is bulk state) is very easy to explode. Since the safety reason, then it is usually stored in a solution in a liquid condition. Pure nitrocellulose is hard and brittle (very easy to crack), then to make nitrocellulose lacquer he needs plasticizers to make the lacquer becomes more plastic and flexible. Plasticizers is a chemical that is very difficult to dry or need very long time to dry. There are many plasticizers materials that are suitable for nitrocellulose, for example: vegetable oil or ester monomers and polymers. Then to develop the character of the resulting film at the finish, into the nitrocellulose lacquer mixture is also added the hard resin materials. There are a lot of hard resins that can be mixed with nitrocellulose, such as: Castor oil, coconut resins, resin, acrylic resin, vinyl resins, etc.

The resin mixture is still need some additive materials such as: surfactants, bubble breaker, flattening powder, etc. An additive material is a material that is needed to make up the performance of the coating. Surfactant is a material to reduce surface tension, make the lacquer flow evenly and make the smooth film. Bubble breaker is a material that allows the film wet longer so that the trapped air in the film formed can be release. Flattening powder is a substance that uses to reduce and adjust the gloss of the resulting film. Finally the lacquer thinner is added into the mixture of nitrocellulose resin, hard resin, plasticizers, and additive to reduce the viscosity and make the nitrocellulose lacquer is in the liquid form. The lacquer thinner is mixture of solvent to reduce the viscosity of the mixture. The special composition of the lacquer thinner is also made according to the lacquer formula and to develop some special properties and character of the lacquer. The thinner mix can be used to do minor adjustment to the lacquer such as: the drying time, the flow, the viscosity.

Nitrocellulose sealer

Nitrocellulose sealer is the same material with the nitrocellulose lacquer, but with different additive. The nitrocellulose sealer doesn’t need flattening powder and lacquer retarder, because the sealer is designed to be sanded and layered with other coating, not as the final coat. Sanding agent the material to help sanding is added to the sealer to reduce the heat arisen by the friction form the sandpaper and the coating. The sanding agent for nitrocellulose sealer is usually zinc stearate or aluminum stearate. This material is the same material used in the sandpaper to reduce the heat during sanding, and reduce the coating material clogged to the sandpaper.

The nitrocellulose coating is very popular in furniture finishing and is still one of the most widely used in furniture finishing industry. The ease in application and the great look resulted make this material is favored by many people in furniture finishing industry. 

The advantages of nitrocellulose clear coating

•   Nitrocellulose is 1 component, very easy to be handled and applied.

Nitrocellulose clear coating is dry by the evaporation of his solvent without any chemical reaction. The film coating formed is easily resolved with his solvent. The film result can be re coated with the same material many times easily. If there any mistake in the finishing, the dry finish is very ease to be washed off and refinish. The drying time is fast at the room temperature, without need a special oven. The touch dry is very fast make his wet coat is not too sensitive to dust or dirt. It can be applied in he simple spraying room and drying room.

• The resulting film layer is very clear, thin and looks natural.

This nitrocellulose coating when applied to wood or veneer surface will always down and become a thin film layer after dry for few days. Therefore, the resulting appearance will look natural and give the natural beauty of the wood. The nitrocellulose clear coating produce the clear film finish with the best clarity, compared with the other types of coatings. Therefore this type of clear coating is the most widely used for high-end furniture products when the appearance and beauty is considered as the most important point.

• The resulting film layer does not emit toxic gases.

Nitrocellulose clear coating will be dry with evaporation of his solvent without any chemical reactions. This clear coating does not need catalyst for drying. Therefore, in his drying process this material depend on how he releases his solvent. It drying times depends to the environment condition. The drying time is very fast, the releases of his solvent takes about 30 minutes in the normal condition. And after it dry, the coating film has no more gas emissions. Therefore, the nitrocellulose coating is quite friendly to the environment. The main ingredient of paint is nitrocellulose, a wood product which is safe for the environment. Of course in the paint mixture is still needed to mix with some certain additives and solvents which may be the pollutant materials, but with the development and the strict regulation the present additive and solvents used is more friendly materials for health and environment.

NC finish for furniture

While the disadvantages of nitrocellulose clear coating

• Relatively low strength.

This clear coating has a relatively low strength to withstand scratches, heat or chemicals. This material is not suitable to finish product which need maximum protection and strength of his finishing. To finish on top dining table, kitchen cabinets or furniture for hotels or cafe, or for outdoor furniture that requires maximum protection and strength for his finishing this material is not suitable.

• The coating film will turn yellow after a certain time

The resulting film layer of nitrocellulose clear coating has a low resistance to ultraviolet light. The film will turn yellow after a certain time, this process is called yellowing. For the dark or brown colors the yellowing is not so obvious, but for light color or white color, the yellowing of the film will affect the final color.The yellowing on the light or white color will change the color and the final look of the finishing. For the light or white finish when the yellowing is not acceptable, usually we need the non yellowing clear coating such as acrylic or butyrate.See our previous article : butyrate coating

Moisture content of the wood

Environment humidity vs moisture content of the wood

Moisture content of the wood

One of the very important conditions of wood that need to be considered in the manufacturing of wood furniture is the moisture content of the wood. Wood is a hygroscopic material that is easy to absorb the water from the environment or release his water inside to the environment. In the room, wood will adjust his moisture content to get the equilibrium with the humidity (moisture content of air) of his environment. And the change of moisture content of the wood will change the size of the wood and create movement on the wood. When there is an increase in the moisture content the wood will expand, otherwise if his moisture content decreases he will shrink. The size of expand and shrinkage is different for each type of wood. The movement is also varying according to the direction of the wood grain.
This wood movement if it is not well anticipated will cause some problems on the furniture products such as: bend, crack, or uneven joint. Moreover the high moisture content of the wood will cause lot of finishing problems, such as: the emergence of the fungus, the adhesion of finishing, cloudy appearance, and many other problems. So the wood needs to be dry, when the processing of manufacturing furniture is started. Especially before the finishing process is started the moisture content of wood need to meet with the standard. Usually 12% is the maximum of  allowed m.c. for wood product.
The wood movement can be reduced with the application for finishing materials especially sealer and lacquer. Therefore, for a big panel such as a wall panel or top table the lacquer or sealer application on both sides is a very important to minimize the change of moisture content of the wood. The sealer or top coat will cover the pores and grain and prevent the change of the moisture content of the wood. However the movement of wood still could happen in finish wood. The finishing layer sometimes is not quite to block the wood from his environment. The best way to make wood furniture is to control the moisture content in the wood. The moisture of the wood furniture as close as possible is controlled in the level with the moisture content of the wood which is equilibrium with the humidity environment where the furniture product will be used. Relationship of wood moisture content with air humidity environment can be seen on the graph between the wood moisture content vs. environment humidity. With the graph we can predict the moisture content of the wood that is suitable for the furniture product that we make and sell. Mostly the moisture content of the wood in the furniture product that sells to the dry country such as US and Europe is between 6% and 12%. While for high humidity country such as Indonesia the moisture content can be little higher.
Freshly cut wood has very high moisture content, after it is left for some time the moisture content in the timber will decrease until it reaches equilibrium with humidity of his environment. This process can happen naturally, but it takes long time to get the wood dry. In the process of furniture production, to speed up the drying time of wood we use oven or kiln dry. The wet wood after being cut is inserted in a wood oven where the hot and dry air is pump and circulated in this room. Since the air is hot and dry, the release of the water from the wood will run faster. The temperature of the air is controlled by a heater. Today the using of kiln dry is already accepted as a standard procedure in the furniture industry. Out from the oven the moisture content wood usually ranges between 5%-7%. Because wood is hygroscopic he will absorb water from his surrounding and the moisture content will increase until reach the equilibrium with the humidity of his environment.
For the production of furniture where the product is use in the same area with the production place, then usually the problems of moisture content is not so much appear, because the water content of the wood used for making furniture has already fit with the environment where the furniture is used. But if the furniture is made to be sold or used in other places where the humidity is different with the humidity of the area where the furniture is made, then the different humidity can cause problems. This problem mostly occurs when the production furniture is done the region with high humidity and the product is sold and export to area with low humidity region. The movement of the wood can be too high, enough to cause construction problem in the furniture. So it is very important to controls the moisture content of the wood in furniture manufacturing and then do finishing process to stable the moisture content of the wood.

Wood properties according to the wood finishing

From the first time when mankind knew furniture until today, wood is used as the main material to make furniture. Wood is the product of provide many advantages. Tree is a renewable material. It can be obtained either  from forests or plantations and with good management the source will always be available. Wood also has unique and distinctive beauty that can not be replaced with other materials. Then it is very reasonable if until now wood is still the main raw material for making furniture.
But to handle the wood properly we need to understanding the wood and its characters. The wood is product from trees which are living beings. Wood has many species that each has different characters and properties.  Wood characters are determined by how the tree alive and growing during his lifetime. The wood with the same species will have the similar character. But since the tree is the living creature, then there will be some variation in properties according to the condition of the weather, soil and place where the tree was living. The with the same type may have different characters when coming from different places.
The characters of the wood must  be recognized by the people who work in the furniture finishing industry. The basic characters of the wood are actually a great value that must be maximized for furniture to make a valuable product. One task of finishing is to maximize the uniqueness of the raw materials that have been there to make valuable furniture. There are at least 4 properties of wood that need to be considered in the finishing process that are: basic color, grains and pores, porosity, and hardness. They are all must be considered by the finishing people, because it will determine the appropriate finishing process for the wood.

• Wood base color

Each type of wood will have different basic colors, such as mahogany wood has smoothly red color, teak wood has a dark brown color, and oak has brown-green color, pine has a yellow white color, etc. For finishing with a solid color or an opaque color, the color of the wood will not affect the finished color because the color finishing is completely determined by the color of the stain used. But for the transparent color, then color of the wood base will greatly affect the final color, because the final color is the combination of the basic colors of the wood and the wood stain used. The wood with light color (white color), can be finished with more color choice. The dark stains can be used to coloring the wood to make dark color. If the light color is chosen, then we can apply the light stain.
For the wood which has dark color then the options for finishing color is quite limited, there is no stain can lighten the basic color. The finish color is limited from the wood base color to the darker. When we choose the finishing color we have to consider the basic color of surface that already in there. The color of the finishing we choose cannot be lighter than the basic color of the substrate. If the final color desired is lighter than the basic color of the wood, the he bleaching process is needed. Bleaching is chemically process to lighten ans whiten the wood. The bleaching process is tough, costly and dangerous so it should be avoided as much as possible. Therefore, the selection of the final finish color according to the base color of the furniture substrates is the best way. Use the wood with the light base color if the light finish color is needed; use the dark wood for the dark finish color. The right selection of the substrate will ease the process of finishing and reduce the finishing cost.

• Wood pores and grains

Each type of wood has a different pattern of grains and pores. Grain is the result of the cambium developed during the growth of the tree. It is mainly determined by the species of the tree and how he lives. Some types of wood have with very strong and clear grain character such as: teak, mahogany, rosewood, walnut, etc. While some other wood have vaguely and soft grain character such as: aghatis, sengon, ash, etc. 
Wood pores are holes that are found in the wood. These pores are also formed when the wood grew during his lifetime and depend to the wood species. There are several types of wood have the big pores such as oak, mindi, teak, etc. While other types of wood only have small and shallow pores such as: maple, ash, beach, etc. But the character of the wood is also depend on how the tree was living. Then the pores and grains pattern of the wood are also depend to the place where the wood come from. Sometime we find the wood with same species have difference grain and pores character. For example we can find some oak wood with smaller and less pores, while at the other time we may find the oak with big and more pores because he comes from different place.
The pattern of wood grains and pores actually is one of the uniqueness and great value of wood that should be enhanced in the finishing process. The finishing should adapt and follow to the pattern of grains and pores in the wood and maximized his performance. The wood with a strong character grains should be proper finish to maximize its unique character, including the grains and pores pattern.

• Porosity 

Porosity of wood is the wood's ability to absorb liquids. There some wood that has high porosity such as: sengon wood, pine, aghatis, etc. They will absorb more finishing materials in the finishing process. This wood will need more coating materials to achieve the smooth surface and film built. Meanwhile at the stain application the wood with a high porosity will absorb more stain and tend to generate the dirty or dark color. To avoid this problem we can use the thin and sprayed lightly in many times. For some types of wood that is really soft, we may need to do the sealer or glue sizing (see our other article : glue sizing for wood finishing).
There also some wood that has very low porosity such as hard mahogany, walnut, rosewood, etc. They are less in absorbing finishing materials and sometimes make the finishing materials difficult to wet the wood.For this type’s wood we also need some modification in finishing process. We can use the slow drying stain to let the stain wet and soak the wood and coloring the pores and grain to maximize the wood finishing color.

• Hardness of wood

Wood also has different hardness depending on the species of the wood. The hardness of the wood is also need to be understood since it will determine the proper process of sanding. The hard wood has the stiff wood fibers that is easy to be cut in the wood sanding. The hardwood is easier to be sanded to produce the even and smooth surface. Meanwhile the soft wood is usually more difficult to be sanded because the wood has more elastic wood fibers. The fibers tend to "lay down" in the sanding process. The softwood need more sanding step. We need to do sanding gradually with less jump grade sandpaper from the beginning to the end. We also need to replace the sandpaper soon after it is blunt and cannot cut the wood fibers
Imperfect sanding will result to the uneven absorption of the stain applied on. It lead to a blotchy or spotty color in the finishing. To overcome this problem for some soft wood we can use wash coat application before the sanding process. The wash coat or sealer will wet and wraps the wood fibers and stiffen them when they dry. Then the stiff and hard wood fibers are easily to be cut in the sanding process to produce an even and smooth surface. See our finishing article : sealer sizing

Sealer and wash coat for wood finishing

Sealer

Sealer is a finishing material that is designed to make a smooth and flat surface finishing after being sanded and coated with top coat. The sealer application should always be followed with sanding process and application of top coat or other sealer. Material to make sealer is the same material to make top coat but with a different additive. Into the sealer there is additive materials called sanding agent to help the sanding process. The sanding agent is a kind a powder that will reduce the heat arising from the friction between the sandpaper and the sealer in sanding operation. The sanding agent will minimized the clogging (sealer stacked at the sandpaper)in the sanding operation.


Wash coat

Wash coat is a sealer that is made thinner than regular sealer. It is made by added some more thinner in the mixture of sealer. The solid content of the wash coat should be at about 10%. The objective of the wash coat application is to form a thin film layer. Wash coat application is widely used in the finishing process that uses glaze. The wash coat application build a thin film layer which is  enough to protect the stain underneath, but is not too thick. The wash coat is designed to let the glaze be able to penetrate and fill to the wood pores and grains. This system will keep the  glaze to enhance the beauty of the pores and grains.

Wash coat is also can be used to take care the soft wood such as: sengon, pine, agathis, etc. Wash coat can be coated at the first step to cover the soft wood. Then, after being sanded, a smooth wood surface is obtained. This process will reduce the "too much absorption" of the wood stain and minimize the blotchy color or spot at the soft wood.
Wash coat application in the finishing process is also help to make smooth surface quickly. Wash coat is a thin sealer that will dry faster than the thick sealer. When wash coat dry, then most of the wood fibers stiff and easily to be cut off with a simple scuff sanding. With a simple scuff sanding mostly the wood fibers off the wood will cut off and cleaned; it helps to obtain the smooth finish surface. Since the wash coat is thin coat, then it dry faster and sanded easier then the thick sealer layer. It will reduce the sanding time at the whole finishing process.

Flow coat for furniture finishing

Sometimes we need to apply top coat some layer. To keep the finishing is not too thick, and make the film resulting is smooth and even, for it is better to use flow coat as the final coat. Flow coat is actually the same material with top coat but designed to give better flow. Flow coat is top coat added with little thinner to low the viscosity. We can add lacquer retarder if necessary to make it slower dry. The thinner top coat will have lower viscosity and make it is easier to be sprayed evenly. The better flow will make even finishing surface and avoid the orange peel and other film problem. Flow coat is also easier in application since it is not heavy as top coat. The slower dry will help to reduce the dust spray especially when we finish big furniture such as cabinet or dining table with wide surface. Flow coat can also be used for revise when the surface was still not perfect such as: a rough surface, gloss is not right, spray line, or other things.

Top coat for furniture finishing

Top coat is finishing materials that applied as the last layer in the finishing process. Top coat is ussually applied over the sealer. Together with sealer the top coat forms film layer in the finishing to cover and protect the finishing color. Top coat is usually applied in a clear state (no color), but it is also possible to applied in a mix with pigment so he has a specific color. The top coat mixed with pigment is called enamel and can be used in finishing process to make solid color (opaque color finish or paint finish). In the solid color finish, the enamel help to make the final color with shorter finishing step. However if we need a good quality finish, the uses of clear top coat is highly recomended. The clear top coat is needed to make the smooth surface film with the right sheen.
In transparent color, top coat should stay clear without any colors. The colored top coat in transparent finishes will make difficulty in controlling the final color resulted. The colored top coat means that the top coat is wished to fulfill 2 functions in the same time, to form the color and to build a film layer.  It makes difficulty to control the result; if we want to get the right color, then we may spray too much or too less top coat which could make the uneven surface, gloss or rough film surface. While if we want the smooth surface, we may won,t get the right color.  For transparent color, the color should be built and controlled by stain application, and the clear coating is applied only to form film layer, then we can get better control for the flow, sheen and the feel.
Some peoples call the top coat with lacquer. Lacquer is actually the paint or resin solution that will dry and make a hard film by evaporation of his solvent. Lacquer is a material that was used as a top coat at the beginning when peoples started using coating to do the finishing. The lacquer was the first coating that replaced the shellac in the early 20th century. With the development of finishing materials and technology now there are lot of materials for top coat. The top coat now is available in many type, made of various resin that dry and cured by chemical reaction that cannot be named as lacquer.
As the final coat, top coat also designed to make gloss of the finishing. Gloss is one of the finishing look that is very important. Gloss is the measurement of the light reflected by the surface. The surface is called high gloss or sheen when the surface reflects large percentage of light that goes to him. A glass or mirror is the example of high gloss surface. While the surface is called low gloss is when the surface reflects small percentage of light that goes to him. Gloss or sheen is measured with gloss meter and usually scale from 0 for the lowest sheen to 90 for the highest. In furniture finishing the sheen of the surface sometime is also called in different way: high gloss, gloss, semi gloss, and semi doff and doff. High gloss is highest and doff is the lowest. There are also some peoples that call the sheen with %, the highest sheen (sheen 90) is called as 100%, while the lowest sheen (sheen 0) is called as 0%.
Lacquer or top coat is made of a mixture of resin with one or more dominant primary resin. The primer resin will determine the type of top coat and form the main character of the top coat. This resin mixture is then solved in his solvent to make fluid form. There is some additive materials are added to mixture to make some special properties that needed. Some additive materials for top coat are: surfactant, plasticizers and bubble breaker. Surfactant is a substance that makes the mixture becomes more homogeneous and will reduce the surface tension, it makes the resulting coating film becomes more even and smooth. Bubble breaker is material affecting the surface tension so that the surface is wet or open longer so make the air trapped in the coating film can be released into the air. Plasticizers is a material that makes the film formed to be more flexible and plastic. It used to make the finishing film is more elastic to prevent cracked in the finishing layer. To make and adjust the gloss of finishing then the top coat is added with matting agent or flattening paste. Flattening powder is made from an inert material such as silica dust that will spread the light in the film layer so that it will decrease the gloss of the resulting film layer. The amount of flattening powder in the top coat will determined the gloss of film formed with the top coat.