Your vintage guitar is resting nicely on its stand when your 3-year-old child crashes into it, leaving a nasty crack right in the nitrocellulose finish. You want to have it repaired, but you don't want to spend a small fortune for a professional luthier to do it. You're in luck, however, because nitrocellulose is the easiest finish to repair.
Mask off areas around the area that needs to be repaired by using light tack tape or blue painter's tape. This will prevent damage to areas that do not need repair. Allow for a 1 inch area around the crack or chip.
Clean all discolouration, dirt and debris from the crack or chip with a fine-point utility knife. It's vital to the success of the repair that the affected area be as clean as possible. If the affected are is not clean, you risk trapping dirt or discolouration under the nitrocellulose and/or the finish might not adhere properly.
Wipe down the affected area with a cotton towel with a light amount of naphtha to remove any wax, grease and small particles. Allow the naphtha to dry before moving to the next step.
Using a hobby brush fill in the affected area until it is level with the surface of the guitar with nitrocellulose. It should then be given time to completely cure. Follow manufacturer's suggestions for this. Several weeks of curing time is not uncommon.
Inspect the area that you filled in to ensure it sits slightly above the surface of the original finish. If not, repeat Step 4.
Lightly sand the new finish (overlapping into the surrounding original finish) of the guitar with 800-grit sand paper until it feels smooth. You should start to see the edges of the new finish blend with the older finish. Repeat this step while gradually moving to 1,000 grit and so forth until you get to 2,400-grit sand paper. At this time your new finish should be the same height as the surrounding original finish and should be very clear.
Use a polishing or buffing compound and a cotton towel to buff the new finish and the surrounding finish to a high shine.
Using water and dish soap when sanding can help you get a better finish on your repair. You'll need to make sure you're using wet-dry sandpaper if you plan on doing this.
Repairing a finish on a vintage instrument can significantly lower its resale and vintage value. If you feel your guitar is worth a lot of money, consider taking it to a trained professional. Using blue painter's tape can harm vintage finishes that may be brittle. Use caution when using tape of any kind on a nitrocellulose finish.