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What will happen if you put a coat of finish on the interior of a guitar. My friend says it will react more slowly to rapid temperature and humidity changes.

It will make it much more difficult to do repair work down the road.

Unless you are coating it in thick plastic finish, you aren't going to slow down any changes that much. Humidity will find its way into the tiny cracks you missed.
Then, at what point will you finish it? If you do so before the plates have been glued to the sides, you have seams that will let in moisture thus negating anything you did.
There's also the problem with it killing the sound. Ideally, a guitar would not have any finish on it for the perfect sound. That's not practical but leaving the inside naked is.
Ad that to Richard's comment on repairs and the answer becomes, "don't do it". Pwoolson38986.5580555556

IIRC Howard Klepper puts a light coat of shellac on the insides of his guitars. Shellac will slow humidity down somewhat, and he does not seem to think it hinders future repairs much.

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Rian Gitar og Mandolin

I am going to start putting a light coating of shellac on the inside of my guitars too.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

I really don't see how puting a light coat of shellac on the inside of a guitar should be any real hindrance to doing repair work. It seems to me that any repair would be just redoing something that has already been done and use the same glueing surfaces that were used before. Besides, scraping a little shellac is not dificult if you need to remove it. I agree also that the shellac will not seal the moisture out but may slow down changes in moisture content. Plus I think it makes for a nice look on certain woods especially if it is padded on over a clean and well done interior.

After all that, I usually forget to do it before I close the guitar anyway and I certainly wouldn't try it after. John How38986.6679861111

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Tickle your guitar daily, and it'll tickle you back.

The other day I installed a rosette on a sitka soundboard (no braces, still flat), and then I put a super-thin coat of shellac on the front to see how the color would come out. (It was nice. ) The next evening was cloudy, and the humidity in my shop went from 45% to 65%. In about half an hour, the soundboard looked like a potato chip. It was clear that the unshellacked side absorbed considerably more moisture than the shellacked side did, causing the soundboard to curl up. The humidity went back down the next day, and the soundboard became flat again.

In luthiery, we tend to break the age-old wood/moisture rule that "whatever you do to one side of a board, do the same thing to the other side." And then, having violated some pretty well-established woodworking principles, we complain about how players need to be much more cautious than they are to temperature and humidity changes.

I believe Bendetto's book suggests putting a thin coat of lacquer inside the soundbox to reduce moisture exchange. It does make future repairs more complicated, and perhaps affects tone as well. Definitely a trade-off.

I had a guitar come in with a cracked side that was finished inside with I believe Shelac. It was a bit of a pain trying to scrape the finish off to repair the crack. That is my only experience.

[QUOTE=Big John How]...After all that, I usually forget to do it before I close the guitar anyway and I certainly wouldn't try it after.[/QUOTE]

Hey John, it's easy. Just take a can of Zinsser Sealcoat, pour it inside, slosh it all around, and pour it back out through the soundhole, port, or trussrod slot etc. E-Z ! ! !



Don Williams38986.6894097222

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

That gives me the willies, thinking of all the dust permently traped to the linings MichaelP38986.696087963

I did put a light coat of shellac inside no 2 before closing the soundbox, just did it for the look, all i know is no 2 sounds great and LOUD but will it affect the wood itself later (trapping moisture inside the wood), i just don't know.

Serge Poirier38986.7466550926

I wouldn't worry about trapping moisture in. You know what they say... what goes in must come out!!

I might try it also. I'm thinking that the only concern is that the outside and inside finish should be applied in similar heat/humidity conditions.

I was thinking about a wipe on poly???

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Joe Beaver
Maker of Sawdust

I know a fairly well respected builder who seals the inside of his boxes (top excluded) with epoxy filler. It looks beautiful.

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Eagles may soar, but weasels don't get sucked into jet engines.

Bill, i really like that idea! Thanks, i think i'll give that a try!

Shelac isn't too bad for future repairs but Epoxy would be murder.

OOOPS, Guess i'll stay with shellac then?

[QUOTE=Brock Poling]
I am going to start putting a light coating of shellac on the inside of my guitars too.[/QUOTE]

Mind if I ask why you made this decision Brock? Is is purely aesthetics?

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Dave Rector
Rector Guitars

No. This is something I picked up in ES's class. As these tops get thinned farther than most people do, and the braces are shaved down even smaller than normal the top is more suseptable to shifts in humidity and this is a way to slow those effects.

It is a safety precaution.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Thanks for clarifying that Brock! I had a hunch it had something to do with Ervin's class.

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Dave Rector
Rector Guitars

Here is what I have seen happen on many guitars. The builder puts a coat of shellac on the in side of the guitar before they put the top on. Then they put the top on and prep for binding, next they cut for the binding, bind and then sand the guitar. Here is what happens. The shellac is still a bit sticky when they start making dust and small bits of wood from the binding cutter and then the bits of wood and dust go inside the guitar and stick to the inside of the guitar. The liners and back braces can get this bad because the soft wood soaks up more shellac and take longer to dry. And the liners can get this stuff in the kerfs and you may never get it out. So if you are going to do this, you should make sure the the shellac is very,very dry before you move on. Just so you know, I have seen this happen on over a hundred guitars. So I am not just making this up. Hope this helps a little.

It does help... thanks for the tip.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

I will honestly confirm what Addam stated, i had dust to remove from the inside, had to use naphta to get it out, just what i could see so it is great advice, thanks Addam!

I second what Paul Woolson said.

Tom

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http://www.moriciguitar.com

If the interior it too finished, i.e. FP, laquer, etc., it can show dust and dirt like CRAZY after it cures, let alone if some should get trapped in the finish itself when it dries/cures.
-j.

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