Wet Sanding Techniques:
Although the great majority of your finish sanding will likely be with dry sanding techniques, wet sanding always has its place in the finishing process. Wet sanding is usually reserved for the final step in finishing, just prior to the buffing process. It is quite often completed with very fine sandpaper, to minimize scratches to make the buffing process much quicker and minimize the possibility of every guitar finishers fear – sanding through the finish.
Scuff Sanding: This is a very light sanding procedure that quickly takes down any high spots in the finish. Be aware though that this sanding is not intended to take down the low, shiny spots on your finish.
Wet Sanding Process:: While water is the usual lubricant that is used in the wet sanding process, you will find that an additional additive to the water will help to keep the sandpaper from clogging and will allow for a faster cutting down of the finish. I often use just a couple of drops of dishwashing liquid to a bowl of water to help reduce the buildup on the sandpaper face.
You will find that by using a very fine paper, the finish will take on an almost glass-like finish and you will begin to see the results of your labor. By using the fine wet-sand paper, you will find that scratches will be minimal and buffing will be amazingly quick.
When in search of a good wet-sanding grade of paper, look for the P-grade papers or a JIS-grade as the CAMI-graded papers will tend to have random over-sized grits that can cause unwanted scratching at this point in the process. While the grits available range between 220 and 2,000, you will find that if you stop at about 600 or 800 that will be plenty good for our processes here. I usually start with 400, progress to 600 and the final step, I use an 800 grid paper. If you plan on using a power buffing setup, such as a buffing arbor, you can more easily remove slightly deeper scratches and you can easily stop at 600 grit.
If you are going to do your buffing by hand, you will want to wet sand with a finer grade, such as finishing with a 1,000 grit or even a 1,200 grit, to practically eliminate surface scratches.
I would not recommend using a power random orbital sanding for this step as the cutting of the finish can be sanded through too quickly and if you do happen to sand through, you will need to apply several more coats of finish.
Also, is you use the soap solution, wash the guitar with clean water several times prior to buffing to remove all residue and left-over grit from the finish. Also even with the fine graded wet sanding papers, you tend to soften the finish just a bit, it is best to let the lacquer cure just a bit prior to buffing. I choose to wait a day or two prior to buffing out the finish. This of course is up to you, but I just feel that with the finish cured just a bit more will give you an even higher gloss.