So, you’ve spent loads of time making the perfect guitar. But before you can buff it to a high gloss, you have to level the finish with sandpaper. Your finish at this point is going to have an “orange peel” look (see picture at left) that will need to be sanded flat. For the sake of this example, we’ll be looking at a catalyzed polyurethane finish on a pedal steel guitar.
Start with 600 grit paper. Resist the urge to wrap the paper around your fingers and start sanding. Using your finger will create uneven pressure and cause your finish to look wavy. Always use some sort of sanding block or backer for the sandpaper. You can make a block out of wood, you can buy a rubber block at most auto parts stores — you can even cut a sanding block out of an old flip-flop. For those with more experience, an orbital or jitter sander can be used as well.
Work in small areas using circular motions. Use very light pressure. Pretend like you’re putting sunblock on your arm. If you rub sunblock on really hard, sorry, I can’t help you.
The idea here is to leave the finish flat with no shiny spots. In the left photo below you can see that there are still some shiny spots that need to be sanded out. Don’t work on those spots individually. Work the entire area until the spots are gone and the finish is flat. It will have a uniform and dull look (see photo on right). Change your sandpaper often and look for “loading” or blobs on the paper. They can leave deep scratches in your finish that will be very hard to remove.
Once the finish is level with 600 grit paper, repeat the process with 800 grit. Work your way up until you get to 1500. With each finer grit, you’re removing the scratches left by the previous paper.
When you’re through the 1500 grit, you should have a level finish and can get ready for the buffing wheels.
– Brian Nutter