Wet Sanding in the Wood Finishing Process

Sanding is a very important job in the finishing process. The fact is the sanding job is half of the finishing work, while the other half is the application of finishing materials. A good sanding process is absolutely needed when you make high-gloss finishing with thick layer film built. The proper sanding is needed to the finish layer every time you are going to layer it with another coat. There are 2 sanding methods in the finishing process: dry sanding and wet sanding. The dry sanding is the sanding when no liquid is used; the heat arisen in the sanding process is addressed by the sanding agent in the sanding paper. You can read about it in our previous article: the sanding to the sealer and top coat.
Wet sanding is a sanding technique that uses liquid to absorb the heat arising during the sanding process. The wet sanding is more used in the finishing process for car refinishes or finishing on the metal products. The wet sanding for wood finishing is rarely used, but sometimes you need to do it. Wet sanding is needed when the thick and smooth finish film with high gloss finish is built. Wet sanding is also done as the first process in the rubbing and polishing. Wet sanding is generally considered a more complicated process because it deals with the liquid, but it can produce a more smooth and even surface.
A finishing process with PU or AC may need the wet sanding to prepare the finish layer before the final coat. The wet sanding is able to produce a smooth surface with less sanding marks, and then a thin layer of the last coat can be layered on without any obviously sanding marks. The wet sanding can minimize the problem of sanding marks and orange peel at the high gloss finish. 

The Preparation of the film layer
 

The wet sanding should be done at the finish with thick film. The thin finish can be sanded with stearated sandpaper as well. The wet sanding at the thin film layer is risky to get the problem caused by the liquid penetration into the pores and grain of the wood. Wet sanding is usually done after the wood surface has been layered with 6 coats of coating or more. The wet sanding can be done as a final sanding to provide a smooth and even surface before the last coat or as the first step in the process of rubbing.
Make sure the coating layer is completely dry when doing the wet sanding. The wet coating film will be loaded at the sandpaper and risk causing deep scratches in the coating layer. The drying time of the finish is varying, depending on the coating type. The NC, AC, shellac, furniture varnish, and water-based coating are usually completely dry after 24 hours, while the indoor PU needs about 1 day or 2 days to dry. The outdoor finish, such as spar varnish or outdoor PU, needs longer drying time.

The materials and tools needed.

•  Machine or hand block sander

Wet sanding is only done at a flat section when the surface finishing requires a perfectly smooth and even surface. The top table or top panel from the cabinets are the subjects of the wet sanding since they are the main focus of the intention of the furniture product. While the other parts, such as the chair, leg, side panel, and drawer, don't need the wet sanding. The small imperfectness at the complex and uneven surface will not be so obvious. The wet sanding at the complex form is also very difficult at the workmanship.
The wet sanding should be done using a sanding machine. The double-pad sander machine is an ideal tool to do the sanding for the big surface, while for small surfaces the hand block can be used.

• Sandpaper.

You need the waterproof sandpaper or the wetor dry sandpaper. It is the sandpaper with the backing and glue that are resistant to the liquids. The waterproof sandpaper is available in very coarse (# 1 grade) to the finest (# 2000 grade). In the wood finishing, usually the sand paper grades from #800 to #2000 are used

• The liquid lubricant.

It is a liquid that serves to absorb heat arising from friction between the coating and the sandpaper. The liquid is also helping the sanding process by smoothing the finish surface and holding the sanding dust. There are some liquids that can be used. Some finishing materials supplier companies have provided a kind of oil as the sanding liquid under some different names, such as cutting oil, rubbing oil, or rubbing lubricant. If the cutting oil is not available, you can use the mineral spirits with a small amount of wax. Water mixed with some soap can also be used with a good result.

How to do the wet sanding.

The wet sanding is about the same as the dry sanding, but you need to wet the surface with the sanding liquid during the sanding operation. Keep the surface always wet when you do the sanding. Put the sandpaper on the sanding machine pad and then move the machine in accordance with the wood grain consistently to the entire surface. Start the first cutting with the coarse sandpaper (grade 600 or 800). Do 2 or 3 strokes of sanding with the machine, and then dry the wet surface using a dry cloth and check the result.

the surface is wet with liquid lubricant 

The sandpaper is attached to the machine pad

 the wet sanding with double pad machine

 Check the sanding result

Watch and observe the sanding result. The sanding is stopped when the surface is well sanded and all orange peel has been truncated. If the orange peel and uneven surfaces still exist, then you have to repeat the sanding until you get a smooth and even surface. 

If you do wet sanding as a preparation for the final top coat, the sanding with the # 600 or # 800 grade sandpaper is enough. The sanding marks will be covered by the final coat well.

But if you want to finish it with rubbing and polishing, then you need to continue sanding with the finer sandpaper. especially when you make a high gloss finish. You can read about the rubbing and polishing process in our previous articles: rubbing and polishing.