Some of them believe french-polishing is the only acceptable finish on concert guitars, others defend many synthetic like polyurethane, nitro liqueur, polyester etc.
My personal preference is polyurethane, but I also give the option of french-polishing on top only.
Lets see, in a very resumed way, the main characteristics of each:
– Very thin finish, does not change or reduce wood vibration.
– Limited protection. As a hygroscopic material, suffers color and texture changes for the contact with skin.
– A thick coat reduces and distorces wood vibration, bringing great prejudice to the instrument sound.
– Great protection. It’s impermeable. With minimum care has enormous durability.
My Personal Opinion
I believe that application is more important then the used material.
Many guitar factories use polyurethane – and others – with the only purpose of finding a beautiful finish. For that they apply many coats of the product. This is the case of a too thick finish.
For musical instruments, what I do is sanding after each aply, until I remove totally the finish from the wood surface. There is material left only in the pores of the wood.
When all the porosity is filled, I apply one only thin coat, ad polish it a few days later.
This is the technique I use in my guitars – high gloss or opaque.
And, as I said before, I have the french-polish option, on top only.