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I think that is a really good point. The same thing that slows down absorption also slows down stabilization. Nothing worse than an instrument that starts changing half way through an outdoor gig. I do most of my playing in the outdoors in the fall of the year with lots of humidity and temperature changes. My guitars - unfinished on the inside seem to do quite well. The only exception was my fault. Played three weekends straight in the rain, took the guitar home and left it in the case for the week. By the 3rd week it was pretty badly corrugated on the top. Took it home, took the strings off, and hung it in the shop for a few weeks and it was fine. Nothing came loose. The top seam does show now, though, but it is still my player. My second guitar.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

I've read Bogdonovich's excellent book on classical guitar building. I notice that he brushes shellac on the inside of back and sides. Given how cleanly he builds, he's definitely not covering up squeezeout. It seemed like he always does this, so maybe contacting him and asking if he believes it makes a difference tonally would add to your data base (again, he's not putting a finish on, just a single, brushed on coat).

He also does a coat on the inside of the soundboard.

I see your point. However, and I'm not sure why this is exactly but, even though I can tell that my guitars with a wash coat inside are moving with RH changes (again more slowly), I find that I only have to make very minor tweaks to the tuning (if at all) when moving to and from extreme RH. My instruments that are bare inside, without a doubt, go much more out of tune overall under the same circumstances.

I'm not telling anyone that they need to convert. I don't think it makes a huge difference one way or the other and obviously there are a TON of really great instruments out there that are bare inside. But, having done it both ways and living in an area that sees wild RH swings, I do prefer a wash coat.

If a panel if finished on one side only, it will move around much much more than if it is finished on both sides. This is really basic woodworking stuff. If anyone doesn't believe it, it's easy enough to try for yourself on a soundboard scrap or something.

That said, millions of guitars have been made with unfinished insides, and they hold up just fine. Obviously then, it isn't necessary, though it might make a guitar a little more stable in humidity swings. Probably the bracing and slight arch are good enough to solve the issue structurally.

On the topic of whether it will change the sound audibly: I actually have seen data concerning the reflectivity of wood with and without finish. It changes much more than I would have suspected, and certainly would be audible. Essentially, high frequencies are more easily reflected by a harder smoother surface. This information came from an engineer who designed folded horns (loudspeakers), in which this sort of data is critical.

Now whether this difference in sound is good of bad is entirely subjective.

I haven't tried using a wash coat so don't know but maybe I'll try it sometime just to see.

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Steve Smith
"Music is what feelings sound like"

This is making me want to try roughing up the inside of my next guitar with some 36 grit and see if that mellows it out.

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Mike Lindstrom

Mike Lindstrom wrote:
This is making me want to try roughing up the inside of my next guitar with some 36 grit and see if that mellows it out.

Try the Rhino coating for truck beds. That should do the trick.

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The dullest pencil is sharper than the keenest mind.