" " wisno wood furniture finishing: How to apply PU top coat.

Thursday, June 20, 2013

How to apply PU top coat.


PU coating is special paint. It is strong, durable and also has nice looked. It is the suitable coating for the product that needs maximum protection. PU is widely used to finish the high end furniture products that require high durability and beautiful appearance. Furniture for hotel, bar, café, dining, kitchen set, and the other expensive Italian style furniture are most coated with PU coating. PU coating has high solid content; he could close the pores and grain to produce close pores finishing pores with high film built.
But instead of his advantages, the PU coating is also known as tough materials. It is quite difficult to be applied, storage and handled. The PU coating is a 2 component materials; it is consist of the coating and the hardener that must be mixed with proper ratio to make dry and hard. The PU coating has short pot life (usually is about 1 hour). It means that within 1 hour the paint that has been mixed will dry and harden. The viscosity of the paint will increase until finally he become gel and cannot be applied anymore. Hence we need to make a calculation about the material amount that we can finish in 1 hour when we mix PU coating.
Another difficulties of the PU is its long drying time. The PU coating that has been layered need long time to dry. It make the PU coating is sensitive to the dust and dirt. PU coating need the clean drying room.
PU coating is also sensitive to the environmental conditions. The PU hardener is isocyanate, the material that is very reactive with water. PU hardener that is placed in open room will soon absorb and react with water from the air to be hard and cannot be used anymore. The PU hardener PU must be stored in closed cans, and stored in dry room to keep it long lasting. 

There some tips to apply the PU coating
  • The clean and dry finishing room.
The PU coating is sensitive to the dust. Any small dust in the finishing and drying room will cause big problem to the PU coating. The PU coating layer is also sensitive to humidity; The PU coating should not be applied in the humid room. Humid room will result in blushing problem and when it is happened we need to sand to clean the blushing part and re coat with new coating. 
Then the drying room also needs to be dry.  The room for PU coating is usually a closed room with controlled air circulation. The intake air is needed to be filtered to keep the inside room clean and dust free. An oven or air conditioning can be installed to control the humidity and temperature of the room.

drying room for PU coating
  • Control of the material
PU material consists of 2 components that have to be mixed in the application. To get the good result then we need to mix the 2 components properly. Read the technical data of the coating and make sure that we do the right mixing.  We also need to check the material condition before use them. It is a catalyzed material that easy to get damage compare with the 1 component material.  The PU hardener is the material that is easy to damage if it is not stored properly. Check the physical properties of the material. The good hardener form in clear and liquid. The hardener that is physically cloudy or too viscous indicate that the material is already in bad condition.
We need to check the viscosity of the mixture, make sure we got the proper viscosity. The required viscosity for spray application is about 12 seconds (measurement with NK # 2 cup). Add thinner if the viscosity of the mixture is too thick. The PU coating that has been mixed will become thicker from time to time, then we need to check the viscosity every time we are going to apply the coating.
  • Good spray technique.
The good spray application is also the important thing to make a good PU coating. The two layer of wet coat is needed to make the even and smooth PU coating layer. The air spray gun is the best spray gun, since it gives the perfect atomization. Air mixed and hvlp spray gun may still be able to be used, but should not be used for the final coat that need perfect coating layer. Some furniture factories or finishing shop prefer to use the cup gun for PU applications. Cup gun with air spray will give more advantages since it give he perfect atomization and also the flexibility to work with small amount coating. 
  • The good sanding between coating layer.
PU coating is thermosetting material. The coating layer cannot be resolved after he dry and hard. The imperfectness coating layer such as: deep sanding mark, scratches, orange peel surface cannot be covered by the coating application on it. We need to do good sanding between coating to get the smooth and even film layer, especially to prepare the surface before application of top coat. Use the fine sandpaper (# 400 of finer); make sure there is no scratch, orange peel or other defect at the surface. We may do wet sadning to get the good surface before final coat application.
 

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