I read a lot of BS about guitar finishes, from why they are used to the characteristics and properties of the popular products.
The primary purpose of any wood finish is to protect the wood, beneath, from excessive humidity (causing swelling and distortion) and desiccation (causing shrinkage, brittleness and cracking.) The aesthetic benefit of showing off wood grain or a lovely stain is secondary.
With guitars, these days, we usually talk about nitro vs. poly. The nonsense surrounding nitro never ceases to amaze me. About a hundred years ago, automobile makers needed a cheap paint to protect the coach work. Nitrocellulose could be applied in a thin coat that was, now read this part carefully, fast drying . Nitrocellulose is a solvent based lacquer, that dries as the solvent evaporates. (Lacquer thinner will dissolve nitro cellulose, please take my word for it.)It is easily cleaned, not dissolved or softened, by naphtha, common ronsonol lighter fluid. (Poly and acrylic finishes are catalyzed, so they do cure, like a two part epoxy and come off, most easily, with a hammer and chisel.) It is dry to the touch in hours and usually dries completely in a matter of weeks, not years. It has no magical powers to let the wood "breathe." (Your wood is dead and air just dries it out.) It is not more "natural because it's made from wood.(It isn't made from wood cellulose, but cotton.) The functional advantage is solely for acoustic guitars, in that it may be applied in a thin coating and so may allow freer vibration of the top. One of vintage nitro's characteristics was a tendency to crack. That characteristic is considerably diminished in modern guitars. Modern formulations of nitro have elastic polymers in them to extend their protection against cracking and modern poly can be sprayed quite thin, so, as owners of recent Epiphones and other instruments know, any differences are barely distinguishable. The environmental/health dangers of nitro are significant in comparison to poly, as well. Poly finishes, especially older ones, can be thick and easily dulled. If applied to wood with even a microscopic spot of moisture, lack of adhesion can cause a milky haze, underneath the finish.
A third and generally "greener" finish option is a simple stain and oil, which can be easily applied by neophytes for a matte finish or polished to a semi-gloss with a little effort.
The more you know.....
;>)/