Official Luthiers Forum!

I'd like to put two coats of shellac on the inside of the guitar im building to help as a moisture (semi)barrier.

What # cut would be best if im looking to brush on two coats.

Also i was planning on doing the inside of the top back and sides, i wasnt 100% sure i should do the top or not so i thought i would mention it and see what feedback i get. I'm doing the assembly on a workboard free hand and im at the point were it would be best to start thinking about getting the coats on there if i want to. top is german spruce back/sides is IRW.

Thanks!

I can't tell by experience, but what I've heard on the subject is that the "moisture barrier" isn't necessary, if you take care of your guitar a minimum. And you must keep in mind that in a way or another, the finish affects the vibrations, and in the inside, it might affect more than on the outside. Especially for the back, who reflects the vibration of the top and return it back. It you put a coat of finish, the sound have to pass throught the finish coats before to get to the wood.

I don't say it's a bad thing... the sound will just reflects differently.

It's all yours!

Francis Richer

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Francis Richer, Montréal
Les Guitares F&M Guitars

I typically do a couple of "wash coats" (1 to 1 1/2lb) on inside B&S (not the top). I use curly maple, so it really pops the grain with what I feel is little downside (in an archtop anyway).

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Dave
Milton, ON

Yesterday, on another forum I read a post from Tom Blackshear - a well-known classical guitar builder. He stated that he applies a thin wash coat of shellac to the inside of the soundboard on his guitars, before he glues on the braces. He feels that this helps to prevent the water-based glues from causing 'imprinting' of the fan bracing through the thin top wood, and also protects the guitar top against sudden changes in humidity.

Whether his rationale is correct or not, I think it's interesting because:
a) he doesn't feel that it degrades the sound quality of the instrument
b) the shellac doesn't seem to affect the gluing of the braces to the top. There are often comments made about wash coats affecting future repairs.

No doubt, a too-thick coat of shellac or too many coats could cause problems.

I'm no expert- strictly amateur- but I now apply a thin (about 1 lb cut or less) shellac coat to the inside of the soundboard (after all the braces are glued) and sides. I'm not very careful with this- I brush it on liberally and wipe off the excess with paper towels. (I saw this first in the Benedetto archtop-building video.) You do have to be a bit careful with rosewood bleeding, so some care is needed there. I often don't coat the back inside as it is glued last and it's difficult to get good coverage without affecting the glue line.

BTW, I think the humidity argument leads to the conclusion that the top is the most important surface to be coated-tops seem particularly liable to cracking in drying conditions.

Cheers
John