Has anyone ever tried varnishing the inside surfaces in the soundbox?
 

I wonder if there would be more effective sound reflection after sanding and application of something more acoustically reflective, would a thin coat of varnish do that?

Eastman applied a coat of something (don't think it was a varnish) to the insides of my E1D - first time I've ever seen that on an all-solid wood instrument. People here speculated that it could have an impact on tone, but might improve response to humidity changes.

It's not all that uncommon actually. I have several '70s era Yamahas that have some sort of finish on the inside. The late Dave Plummer finished the inside of his guitars, and I know there are others.

Somogyi uses that technique on the inside of his guitars though I believe he prefers to use French polish for that.

Yes, I've also seen that on some (though not all) of those late 70's MIJ Martin copies by Matsumoku, et al. My current "beach guitar", an Aria, has that; the back & sides are laminate, so I couldn't say if there's any advantage to it.

FYI, French polish is a technique, not a material. The material used is shellac.

The people that do it use shellac but it is not an accepted theory in the lutherie community

I did a thread on this years ago, and there some notable guitar builders who do it.

I always understood they don't seal the interior of the instrument to allow for changes in humidity.

Ed

I am guilty of being careless there of using the term as being synonymous with shellac but i am in fact aware that french polish is actually the technique of applying shellac.

" varnish inside guitar"
 

Ervin Somogyi and at least 2 of his prior apprentices Matsuda and Ebata finish the inside of the guitar with appears to be lacquer. I've owned 2 Somogyi's 2 Matsuda and ! Ebata (for sale at Luthier's Collection) and all of them appeared to be " sprayed " with lacquer. I guess the reasoning is it would improve the reflection of the sound to the area of least sonic restriction ( the sound hole). The sound of all three of the builders was spectacular , but I've played many guitars that sounded as good.

Hi, I used to have two '70s Daion guitars which had some sort of semi gloss polish. They were very bright.

Way back when I was a drummer I wrote (a letter!) to Premier Drums to ask them about paining the interior or the shells and we had quite a dialogue about it, then I painted mine. Can't remember much difference.

I'm not a luthier, but may have something to contribute here.
When I was building my first.. and only so far.. guitar, I asked my mentor about sealing the inside of the box.

His response had two parts:
1) "Do you remember before we started, I told you to do as I show you.
Don't try to reinvent the process. The way I do it has worked for a long time.
Leave it be."
2) "Now, that said, if you want to seal the inside of a guitar, do it.. on your NEXT guitar.
Do whatever you want, but not this time.
And don't try to 'sell it' to me."

He knows what he's doing.
He has over 50 handmade guitars to his credit, lovingly built one at a time.
They play and sound wonderful.

Still, I'm strongly considering putting some kind of sealer inside the one Im starting this fall.
Not exactly a gloss finish (that may be sonically reflective), but something to reduce moisture absorbing into the pores.
We'll see.

To clarify a few misunderstandings in this thread:

1) Somogyi and his apprentices seal the insides with a thin coat of shellac. The thinking is that this will slow down rapid humidity changes.

2) Sealing the insides with shellac is not that unusual. A number of classical builders do this. Somogyi started out as a Flamenco guitar builder, so he is just following in that tradition.

3) Fine sanding of the insides should yield a similar result as sealing/finishing the insides of the guitar. I do this on my instruments (sand to around 600 grit) but I do this for looks rather than a belief that it improves tone.

4) The only negative for finishing the inside of the guitar is that it can make subsequent repairs a bit more difficult (regluing a brace for example). However, a thin coat of shellac is not that difficult to remove.

The main reason I don't apply shellac to my interiors is that I feel it yields an unattractive appearance. To my eyes it just makes the insides look sloppy and this is largely because the grain is raised during a 1st finish coat. Normally, you'd go back and sand this down and apply several more coats until you have a nice, smooth surface but no respected builder wants to add anything more than a micro-thin coating to the insides - remember, the purpose is to slow down humidity transfer and also, thick finishes are the enemy of good tone.

There does not seem to be any evidence or consensus that a sealed inner surface would help to contain moisture in the wood. The common finishes used in guitar making are simply not impermeable. A UV cured poly finish might be vapor tight, but nitro and shellac are not. Secondly, fine sanding and sealing the inner surface might slightly affect tone, but only for frequencies that your dog or cat could hear - far above the range of human hearing (20 Hz - 20,000 Hz for a healthy ear).

In short, it would be a lot of extra work for no real benefit.