To be clear "gillymor" (note spelling) did not write, "let thick varnish get out of the way and let spruce do its work". In a Bluegrass jam volume is essential because you're trying to be heard, and even hear yourself, amongst a bunch of banjos, mandolins and fiddles and not all of them quiet down when you take your solo so volume is important in that context and tone takes a back seat. Luthiers use the various kinds of finishes to alter and hopefully enhance the tone of an instrument and sometimes select the type of finish to complement the characteristics of a particular instrument. I have some fine hand-crafted instruments and would not think of sanding them down to raw wood. Here's some comments from Dana Bourgeios on the subject (I'm lucky enough to own one of his fine guitars):

"The trick is to select a finish that best compliments a guitar’s design and voicing considerations, and to apply it in a way that damps undesirable frequencies, but doesn’t damp desirable ones."

"I try not to judge the relative merits of varnish, lacquer, urethane, polyester, and other finishes, for much the same reason that I cannot say Adirondack spruce is superior to European spruce, or that Brazilian rosewood is always a better choice than mahogany. Like tonewoods, a finish can only be judged in the context of a specific application."


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