Applying a coating to the exterior of an instrument has always been thought of as one of the most important aspects of the process of building a guitar. It not only adds to the wood depth and beauty but also protects it from moisture. Lastly, it has an effect on the sound of the instrument by dampening some of the harsh frequencies.
Most makers of guitars agree that it sounds better when the finish is applied as thinly as possible. Too much finish and you can dampen the sound excessively, loosing some of the depth of the sound.
The traditional way of finishing guitars has been to apply shellac by rubbing the surface of the guitar with a pad almost dry of the solvent, alcohol, creating a pressure feed that also produces a lot of friction, thus polishing the surface at the same time as one applies a minimal amount of material.
This technique has the additional benefits of being environmentally friendly and also less toxic for the finisher than are modern finishes. Shellac is a product that comes from an insect excretion, harvested and refined in an ancient and natural way. The solvent is ethanol or pure grain alcohol, the filler is ground pumice, and the lubricant is oil.
The result is a beautiful finish of great depth without being thick. It is true that a french-polished instrument does need to be cared for, but this is true for any hand-crafted guitar, no matter what the finish. The french polish can always be touched up, and it develops a nice vintage patina as it ages.