Why I use a Shellac Finish on my Custom Guitars
The reason that shellac makes such an exceptional finish for custom guitars is that it is relatively hard and thin, and absorbs less of the guitars energy than a nitro finish.
Add the facts that the finish is infinitely repairable and that the solvent is relatively benign (alcohol), and you have an equation that's pretty hard to beat.
In fact, Shellac is slightly too brittle, as a thin membrane of it will crackle during normal wood movement. For this reason I use a finish more properly termed a "spirit varnish", composed mostly of shellac with mastic and sandarac resins added in small amounts to promote flexibility to the correct degree.
This is not my recipe or concept, I am borrowing it from the violin world, which has developed a number of time tested recipes for spirit varnish (different from oil). It works very well. It can be sprayed on in order to build a finish thicker than can be achieved by french polishing, lending the finish more durability. Even though it is noticeably thicker, the finish still comes in under 0.010" thick.
Although I use a spray gun to apply the bulk of my finish, I do use the french polish technique to fill my pores (1st step), and as a final rub out. French polishing during and after the pore-filling will fill and seal the wood and allow you to spray on a layer that forms a continuous membrane within one or two sprays. Otherwise the finish will just keep showing pores.
My intention is to duplicate the brushed-on spirit varnish finishes that died away as soon as lacquer became king. I have seen hundred year old specimens of these finishes that appear to be in quite good condition. Why not imitate a great idea?