How to sand sealer and lacquer in finishing process

Sanding is very important in the finishing process for wood furniture. A good and appropriate sanding is always required in the finishing process either in the preparation of unfinished products, and also at the finishing process. The explanation about the sanding to the unfinished product can be read at our previous article: how to sand the unfinished wood furniture. And in this article we want to discuss about the sanding process in the finishing process.
Sanding in the finishing process is the sanding to the wash coat, base coat, paint, sealer, lacquer, coating and other finish material that built film layer. Most modern finishing is the coating and they are applied by spray gun. The finishing process to build the film layer always needs proper sanding to make a smooth and even finishing surface. The good sanding is also needed in the finishing process when the material application is done by brushing or wiping such as in the application of shellac or varnish.

Unless the final top coat, every layer of clear coating (sealer and lacquer) always needs a good sanding before go to the next process. An inappropriate sanding will cause problems in the finishing qualities such as: orange peel, rough, uneven gloss or adhesion problems in the finishing layer. Sanding for sealer and lacquer should be done by using stearated sandpaper (sandpaper which is equipped with a sanding agent). Sanding agent in this paper can reduce the heat arising from friction between the clear coating and sandpaper and help to avoid the finishing material stick to the sandpaper (clogging). The clogged sandpaper is high risk to create deep scratches on the surface finishing. 
 
The grade sandpaper used is range from 240 to 400 (USA standard) depending on the type of clear coating is used. To sand the clear coating that dry only by evaporation of his solvent then the sanding process is more simple, because the clear coating layer underneath will resolve when coated with another sealer or lacquer. They are the shellac, varnish and lacquer. The nitrocellulose clear coating is the easiest material in term of sanding process. It is fast dry coating and does not need a perfect sanding. The top coating layer will wet and resolve its underneath layer and bend together. The small sanding mark will resolved and the even smooth film finish will be obtained. The sanding with 240 grade sandpaper or 280 at the sealer is usually enough to produce the smooth finish film layer.

But the better sanding is needed for application of the "chemical reaction coating" since the film will not resolved by solvent. The polyurethane, acid curing, catalyzed lacquer, precatalyzed, water based coating are need the sanding with fine sandpaper to make a smooth and even surface. If there any deep and big sand mark they will be appeared at the final finish layer. The fine (grade 400 or 380) sandpaper is needed to avoid the sand marks appearing in film finished. The water based is not catalyzed clear coating, but it require good sanding process since the water based coating new layer won,t dissolve its underneath layer.

The task of sanding has a slightly different between the first sealer and the second sealer or top coat. In the first sealer while film layer is still thin, then sanding is intended to cut the wood fibers that is raised and stiffed by the sealer application. Therefore when we sand it must be ensured that the sealer is already dry, to make sure that all the wood fibers are stiff and can be cut off with the sanding process. For the purpose of easier cutting the wood fibers we can use a wash coat as the fist sealer. With the application of a wash coat (thin sealer), then most wood fibers will be stiffen and dried faster, and then with a simple scuff sanding mostly the wood fibers are cut off. Since wash coat is a thin layer it dries faster and then easier to be sanded than a thick layer of sealer. After the most of wood fibers are cut off, then a smooth surface finish can be obtained faster.

Sanding on the second sealer or top coat layer is intended to make the film became more even and to give good bond with the top layer. the sanding is also need to be done well to clean the orange peel or uneven surface. The sanding is must be done when the film layer is dry. The wet clear coating film tends to stick to the sandpaper (clogging) and it is very risk to cause deep sanding scratches on the film layer. When the thick enough film layer has been already built, the sanding machine can be used to accelerate the sanding process, especially the flat surface. But however it must be be done carefully to avoid the sand through (too much sanding until cut off the finishing layer).
To sanding the uneven shape, such as carving, curved shapes, profiles, rod form, the sanding should be done with a more flexible abrasives like: scotch brite, sponge sanding or steel wool.

sanding with sandpaper by hand

Sanding the sealer on flat surfaces with sanding machine

Sanding the sealer on carved surfaces using scotch brite