Most of the time the client will give up and ask that I refinish the guitar with a more durable material. I have invariably found that the sound is unchanged by this transformation. If the client can hear the difference and really wants it though, I do it. Perhaps my ears are not as good as theirs. Or perhaps my practicality out weighs my sense of romance after days of rubbing and sore fingers and shoulders.
Varnishes are another early creation of man.
I believe that the Chinese had developed them many centuries ago and created art forms like lacquerware around them. The formulation of some of these creations were truly kept secret and are not well known. The resin in the finish is created by allowing the turpentine to polymerize. When allowed to evaporate, turpentine will polymerize into a thick resinous goo. If then spread out it will dry into a brittle film which requires some plasticizer to add flexibility to the film.