How to Apply PU Top Coat for Wood Furniture

PU coating is special; it is strong, durable, and also has a nice look. It is the best coating for the furniture product that needs maximum protection with a good-looking finish. PU is widely used to finish high-end furniture products that require high durability and a beautiful appearance. Furniture for hotels, bars, cafés, dining, kitchen sets, and other expensive Italian-style furniture is most commonly 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, stored, and handled. PU coating is a two-component material; it consists of the coating and the hardener that must be mixed in the proper ratio to make it dry and hard. The PU coating has a short pot life (usually is about 1 hour). It means that within 1 hour, the paint that has been mixed will dry and harden. Within his pot life, the viscosity of the paint will increase until finally it becomes gel and cannot be applied anymore. Hence we need to make an accurate calculation about the material amount that we can finish in 1 hour when we mix PU coating.

Another difficulty of the PU is its long drying time. The PU coating that has been layered needs a long time to dry. It makes the PU coating sensitive to dust and dirt. PU coating needs the clean drying room to minimize the dust and dirt problem.

PU coating is also sensitive to environmental conditions, especially the humidity. The PU hardener is isocyanate, the material that is very reactive with water. PU hardener that is placed in an open room will soon absorb and react with water from the air to be hard and cannot be used anymore. The PU hardener must be stored in closed cans and stored in a dry room to keep it long-lasting. 

There are some tips to apply the PU coating successfully

• 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 a big problem for 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 problems, and when it happens, we need to sand to clean the blushing part and recoat with a 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.

• Control of the material

PU material consists of 2 components that have to be mixed in a proper ratio 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 using them. It is a catalyzed material that is easy to get damaged compared 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 forms in clear and liquid. The hardener that is physically cloudy or too viscous indicates that the material is already in bad condition.

We need to check the viscosity of the mixture to 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, and then we need to check the viscosity every time we are going to apply the coating.

• Good spray technique.

A good spray application is also an important thing to make a good PU coating. The two layers 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 a hvlp spray gun may still be able to be used, but should not be used for the final coat that needs a perfect coating layer. Some furniture factories or finishing shops prefer to use the cup gun for PU applications. Cup guns with air spray have advantages since they give perfect atomization and also the flexibility to work with small amounts of coating. 

• The good sanding between coating layers.

PU coating is thermosetting material. The coating layer cannot be resolved after it is dry and hard. The imperfectness of the coating layer, such as deep sanding marks, scratches, or orange peel surfaces, cannot be covered by the coating application on it. We need to do good sanding between coatings to get the smooth and even film layer, especially to prepare the surface before application of the 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 sanding to get the good surface before the final coat application.