After a good deal of wear or abuse some areas of a finish may wear through exposing the lighter-colored wood. As long as no stain, glaze or toner is involved, you can usually fix these problems simply by applying more finish on top.
The easy test to see if this will work is to apply a little liquid to the wear-throughs with your finger. You could use mineral spirits (paint thinner) on any finish without causing damage, but the most convenient liquid is from your mouth.
Wet your finger and wipe the liquid over a small part of the surface, as I’m doing in the accompanying picture. If the color comes back, as it does here, all you need to fix the problem is more finish.
The surface should be completely clean before you start. All finishes will work, but be careful of lacquer because the lacquer-thinner solvent is strong enough to blister and lift many finishes, including an original lacquer finish. If you are using lacquer, apply very thin, quick-drying coats, or apply a washcoat of shellac first.