Combination of some types of coating in a finishing process

Basically, the finishing process involves the application of two types of finishing materials: stain to form color and coating to form a film layer. There are several types of coatings available to form a film layer on wood finishes, such as NC, PU, ​​melamine, UV, vinyl, polyester, acrylic, and water-based, each with its own advantages and disadvantages. Ideally, a finishing process should use only one type of coating because each layer will blend perfectly, ensuring a safe system. However, there are some situations where a specific color and finish appearance require a combination of several types of coating to achieve the desired color and appearance. Using these various types of finishing materials can be a better option as long as it is done correctly. Here in this post, I will share tips and tricks for completing the finishing process with several types of finishing materials in one finishing system.

Wood Stain Application

Wood stain is a thin, color-forming material primarily used to color wood and create transparent colors. The main components of wood stain are pigment and thinner, with a small amount of binder. Woodstain doesn't form a film, so it can be used in a variety of combinations without significantly affecting the performance of the resulting film as long as it's completely dry.

Wood stains can be divided into three types based on their solvent type: solvent-based, oil-based, and water-based. It is very important to make sure every layer of wood stain is completely dry; no solvent remains on the surface when the next coating is applied. This issue may not be very crucial when you use the same type of thinner for the stain and coating. For example, if you use the waterbase stain with the waterbase coating or the solvent base stain in the solvent base coating system, the coating and the stain will release the same solvent and dry together. 

But when you use different solvents for the stain and the coating, then the drying of the stain is very important. When you apply a water-based or oil-based stain in the solvent-based coating, then make sure the stain has dried completely; no oil or water remains on the surface when it is layered with the next coating. The oil or water cannot solve the coating; it can be trapped between the coating layers and affect the fusion of the coating layers and lead to coating problems such as adhesion issues, coating cracking, and other issues related to the strength and durability of the finishing layer.

For simple information about the wood stain application, you can view the infographic image below

Glaze Application

Glaze is a stain applied between coats. Glazes come in only two solvent types: oil-based and water-based. Oil-based solvent glaze is generally designed to be applied together with solvent-based coatings. Water-based glaze, on the other hand, is designed to be applied with water-based coatings. In theory, water-based glazes can be applied in the water-based coating system, as long as the glaze is completely dry before being layered by the coating. However, in practice, this is rarely done due to application difficulties. Water-based glaze tends to dry fast and is more difficult to be applied than oil-based glazes. Applying water-based glazes over solvent-based coatings usually makes the application process more complicated.

Conversely, oil-based glazes are rarely used over water-based coatings due to application challenges. The oil solvents in oil-based glazes can easily damage water-based coating layers, making application very difficult. The slightest mistake can risk damaging the underlying water-based coating layer.

Coating Combinations

Coating refers to a finishing material that forms a film. This film determines the strength of the finishing layer and provides protection for the finished layer. Several types of coatings are commonly used in wood finishing processes: NC, UV, PU, ​​AC, vinyl, acrylic, and water-based. Each type of coating has more complex properties depending on the type of coating and resin mixture used.

Some coatings are clear, meaning they are colorless, and include top coats and sealers, which serve to coat the finish without creating a color. There are also coatings mixed with pigments to create color. These coatings not only form a film but also create a color for the finish. There are 2 types of pigmented coatings: base coats or primer (pigmented sealer) and enamel (pigmented top coat). 

Combining coatings is more complex and must be done correctly to prevent problems. Each type of coating has different characteristics that must be considered when combined in a finishing system. Incorrect combinations can quickly result in the failure of the finishing layer.

Basic rules of coating combination

In general, there are several rules that must be followed when combining several types of coatings.

• Thermosetting coatings can be coated with any coating.

A thermosetting coating is the coating that is not dissolved with thinner or solvent after it dries. They are coatings that dry through a chemical reaction, such as polyurethane, acid-curing (melamine), UV, polyester, epoxy, and 2-component water-based coating. When this coating layer dries, it becomes thermosetting, strong, and hard and won't be dissolved by thinner. When this coating layer is re-coated, it remains intact and does not dissolve, making it safe to be coated with any coating.

• Thermoplastic coatings cannot be coated with coatings that dry through a chemical reaction.

Thermoplastic coating is a coating layer that will re-dissolve when exposed to solvents. This is a single-component coating whose drying process occurs solely by the evaporation of the thinner without any chemical reaction. These coatings include NC, vinyl, acrylic, or butyrate; shellac; and other single-component coatings.

This type of coating will dissolve and melt when coated with a coating above it. The underneath coating will blend with the coating layer applied above it. Therefore, this type of coating is not recommended to be layered with coatings that undergo chemical reactions. This layer of coating can mix with the coating above it, disrupting the chemical reaction. This reaction can pull the coating layer underneath, causing problems such as cracking, peeling, or shrinking.

• Thermoplastic coatings can only be coated with compatible material

Ideally, thermoplastic coatings should be coated with the same type of coatings to avoid the risk of problems resulting from incompatibility between the coating layers. However, some thermoplastic finishing materials can also be used together. Vinyl coating, for example, can be used in combination with other coatings, such as NC or acrylic coating. 

One-component water-based coating is actually not a real thermoplastic material. It can be wiped off or washed with solvent and thinner, but the coating does not melt and resolve. The water-based coating can mostly be combined with the solvent-based coating, such as NC, vinyl, and acrylic, with good results but must be carefully done. Avoid too thick of a solvent base coating on the water-based coating since it may affect the look and strength. Applying the "reaction coatings" (PU, melamine, UV, PE) on the "non-reaction" water-based coating is not recommended; the reaction of the coating may attract its underneath water-based coat.

For a detailed combination coating recommendation, you can view the infographic images below

Things to be considered when you develop finishing process with many coating types

• The drying of the coating layer

Coating that hasn't dried will continue to dry until it's completely dry. If the coating dries with the reaction, it will continue to react until it is fully dry. If it is a "solvent dry" coating, then it will continue to release its solvent until there is no more solvent in the coating layer. If the undry coating is layered with different types of coatings, the drying processes of them may not be well integrated. Moreover, the undry coating has not reached its maximum strength, so it may not be strong enough to hold the paint above it.

Therefore, ensure each layer of coating that will be coated with a different coating is completely dry before proceeding with the next process.

• The proper sanding

Every dried layer of coating always requires a thorough sanding before being layered with another coat. Sanding is purposed to even out and smooth the surface and to create scratch marks to form bonding between coatings. For the thermoplastic coatings finish, slightly imperfect sanding may not be a big problem, since the coating layers will dissolve and merge. However, in the thermosetting coatings, the dry coating will not dissolve; then the sanding process is a very important step. The good sanding is purposed to create scratches as a bond between layers of the coating. The missanding at some surface will cause imperfect bonding between the coating layers.

• Test and trials

Whenever you perform a finishing process with a new system, it is highly recommended to conduct trials and tests before running this system in production. Basically, you can always do new things to explore the finishing system and materials, and you may discover new experiences and results you never knew before. However, before running a finishing process for a big product or big-volume production, you have to make sure that your system works well. Make a panel with the system and do the necessary tests to see the finishing performance.