This text is not a how-to-do-it description. It's much better to see it than to read about it. I recommend that you watch a demonstration of french polishing on the video Hand-Applied Finishes: Applying Top Coats by Jeff Jewitt (published by www.taunton.com). This video shows french polishing a table top. The well made video French Polishing for Guitarmakers by Ron Fernández (available from www.fernandezmusic.com or www.stewmac.com) explains in detail how it is done on the object we are interested in.
French polishing is the quite work-intensive technique of applying shellac with a cloth pad. This very old finishing method gives particularly good results on mahogany, but also on other types of wood.
In French polishing shellac (thinned with denatured alcohol) is applied by moving a pad (made of lint-free cloth) in sequences of circles and figure eights, using a bit of oil as a lubricant. French polishing has a reputation of being very difficult - apparently only experts are capable of doing it. I don't agree. The first time I tried French polishing I was very pleasantly surprised by the results I achieved and watching the gloss slowly develop was a very satisfying experience indeed. To me French polishing is a great and extremely rewarding technique - so why not give it a try?
Dissolve the thin shellac flakes by simply pouring them into alcohol and shaking the solution repeatedly before leaving the flakes to dissolve overnight. Always prepare only the amount that is needed for a particular job as it will not keep very long. A simple test to show whether old shellac can still be used or not is to leave a small drop of the solution to cure overnight; if it doesn't cure but remains gummy, pour it away. Different shellac-alcohol mixtures are commonly used: a mixture of a 1/4 pound of shellac flakes dissolved in 1 pint of alcohol is called a "2-cut" solution.
Shellac is not affected by hand sweat and can withstand temperatures of up to about 80 degrees Celsius (176 degrees Fahrenheit). It is, however, quite easily damaged by alcohol. Surfaces thus damaged are fairly straightforward to repair, though. Avoid using alcohol-based stains under a shellac topcoat as this will cause the stain to be dissolved by the alcohol contained in the shellac mixture, which will lead to patches forming on the surface.
Excerpt from my book Building Electric Guitars