In the long run, finish thickness does make a tonal difference. But since neither of our theories can be proven...
Believe it or not, there are many, many guys out there with many years (likely more than you) under their belt who agree with the "thin skin" myth. Means about as much as your belief in the contrary, I guess.
If they have more years "under their belts" than me, I pity them. I've only been working on these things for over forty years.
The lab proof, I believe, would be effectively impossible to obtain because of the impossibility of constructing adequate and effective controls in the protocol.
Regardless, the claims for audible sonic superiority of a thin finish on a solidbody are inherently nonsensical on their face as they defy logic, and their roots lie in misattribution.
What do I mean by that?
This superstitious belief has its muddled origins in the history of classical acoustic string instruments.
On a world-class violin with a top carefully carved from a single piece of wood and "tuned" by adjusting points of thickness on a very thin, intensely vibrating top, the thickness, rigidity and makeup of a finish can and does make a measurable difference in the way the top vibrates and consequently how the violin sounds. It affects the top's material consistency and to an extent even its mass.
Still, even here, these effects are somewhat subtle.
To a substantially lesser degree, these same concerns apply to a fine classical guitar, which has a top of relatively lightly-braced,
very thin,
very light,
very soft cedar or spruce vibrating under relatively low tension. Such a top bonded to an extremely hard ~.006" layer of catalyzed polyurethane finish has been substantially altered in its makeup and vibrational properties.
A trained ear may detect a noticeable difference between a comparable batch of guitars with tops finished this way rather than in another, as the tops are very closely graded for consistency in the production of fine classical guitars.
With steel-string acoustics, this situation is still present, but has diminished even further as the guitar top is more rigidly braced and under far higher tension so the structural characteristics of the finish are proportionally less important to the vibrational characteristics of the top.
So, the importance of the finish is meaningful on these guitars where the sound is produced by a thin membrane of coniferous softwood. A classic Cremona thin varnish may produce, to the trained ear, a more complex tone than a thick layer of polyurethane.
All right. With me so far?
OK, while we've been talking, Bubba has arrived lugging his Peavey T-40 and has been trying his best to follow all this.
At this point, he says to himself, "Dang! I'm gonna go home and strip them polyurethoodles off and get me some of them cremonies and Ol' Betsy gonna have AWESOME
TONE!"
He can barely contain his excitement, though -- predictably, being Bubba and not real good at this
thinking business -- he errs. Worse, he's typical of his breed, gristlehead gear forum trash, so he will disseminate his dire misconceptions to his fellows worldwide by nightfall. He will laboriously trash Ol' Betsy and will offer to fight anyone who suggests she does not have improved
TONE as a result of all his arduous and destructive scraping.
Me: Class, in which errors do Bubba's tragic folly lie?
Billy: Bubba isn't playing a fine classical guitar. He's playing a whacking great boat anchor of an electric bass which relies for its sound
primarily upon electromagnetic transduction and downstream signal modification and amplification.
Gordon: [rolling his eyes] Never mind the isolation of the body by that anvil of a high-mass bridge anyway.
Nigel: To expand upon William's point, even if this...um...Bubba chap was considering unamplified results, he is not dealing with a delicately vibrating wafer of fine spruce weighing an ounce or two, but a body consisting of some frightful, six pound hunk of ungraded furniture ash. I should imagine it would make relatively no difference if he dipped the wretched thing in road tar.
Molly: Bubba is a waddling swine who weighs twenty stone if he weighs a bloody ounce...
Me: Language, Molly!
Molly: Sir. When he plays that thing, he wraps his sagging fat around it until he looks like an amoeba taking his feed. If anything touching that bass could vibrationally damp it, such extensive contact with Bubba's foul, wet self should, I reckon, be like having it played whilst jammed into a vat of axle grease.
Me: Splendid, and full marks to you all! I thank you for reinforcing my decision to teach in a decent prep instead of an odorous sink school full of chavs and schemies!