" " wisno wood furniture finishing: How to spray top coat.

Saturday, January 21, 2012

How to spray top coat.

Top coat is the top layer of a finishing. As the top layer, he functions to produce a smooth and even surface. The spray application is the best way to layer the top coat. Other application method such as: brushing, rolling or padding can be used, but the result will not as best as spray application. The spray application is doing 2 operations in the same time:  the material atomizing and the layering to the surface with fully control. With the proper technique we can get the perfect finish layer.
But to be producing a smooth layer of top coat, the spray application must be done properly. The top coat is applied at the last process, so a perfect coating is needed. The imperfectness at the top coat application will result on a bad quality finish. To fix it, we have to re sand the top coat layer and apply another layer of top coat with good technique.



spray the top coat

Here are some tips to spray top coat in the finishing process
1. Good preparation for the underneath finishing layer.
The top coat is the top layer at the finishing; it is usually applied after few layers of finishing material are coated to the wood. Some layer of sealer or paint is usually used as the previous layer to build the film layer underneath. To produce a smooth finish and even top coat, we need to well prepare the film finish underneath. The good sanding is very important thing to prepare the film. For finishing with a low gloss and open-pore or antique looked, some imperfections may still be acceptable, because it will be hided with the finishing looked. But for finishing with clean looked, high gloss and close pores, then the perfect preparation is required.  
Make sure the whole surface is sanded well. And no orange peel or rough surface at the layer. If we used the non reaction coating such as : NC, lacquer, varnish or shellac then we can use # 320 or # 360 grade sandpaper as the last sanding. The non reaction dry film will be melted and resolved with the top coat application. But if the thermosetting dry coating such as: PU, AC or water based coating is used, the better sanding is needed. The sanding mark at the film could be visible in the final finishing looked. We need to sand with # 400 grade or finer to make sure there is no big sanding mark at the finishing layer. Some time we need to do wet sanding to get the smooth and even layer. You can see our previous article: how to sand sealer and top coat  and wet sanding for the wood finishing
2. Good preparation for the finishing material.
Top coat is a finishing material which has several functions at once; form a smooth surface, build the sheen and provide enough protection to its underneath finish. To get all its function is fulfilled then we need to prepare well the top coat. Check the type of the top coat; make sure it is compatible with the underneath layer. Check the sheen of the top coat and the condition of the top coat.
The top coat should flow well when it is applied. Stir well the top coat and check the viscosity of the material; make sure he meets the required viscosity for the spray application. The viscosity around 12-13 seconds (with the NK 2 cup) is needed for air spray application.  If the material mixture is too thick, then we need to add some thinner.  Use the thinner that is recommended by the supplier of the top coat. To spray big case then we may need some retarder to slow down the drying time and let the top coat layer wet longer to make smooth surface.
3. Good preparation for the equipment and room facilities.
The air spray gun is the best equipment for spray top coat. The other type of spray gun such as airless, HVLP or LVLP can be used, but the best result is still the air conventional spray gun. The air spray gun gives the perfect atomization and best control to the top coat layer. Clean the spray gun, make sure it works properly. The good preparation for the finishing room is also needed.  Make sure the finishing room is clean and dust free. The existence of dust can be attached to the top coat and result for rough surface finish film.
4. The correct spray technique.
After all preparation is done well, then we need to do the correct technique. Set the spray gun in the right way, the material volume, the air pressure and the spray width to get the perfect atomization (see our previous article: How to use spray gun correctly 1). Then move the spray gun with the right technique (see our previous article: How to move the spray gun). Make sure a wet and even layer coat is obtained.  Then place the wet coated product to the clean drying room to let it dry.

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