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I believe there is another big player in the effects of RH changes on our instruments. The finish on the outside does a lot more than just look pretty, show off the grain of the wood, and protect the wood from abrasion. It slows down (to greater or lesser extent depending on the finish) the transmission of water molecules into the wood fibers. Given this, why don't more builders finish- with at least a wash coat of shellac- the inside of the guitars. My engineering training made this make sense to me from the get-go, so I've used it in most of my guitars. I was gratified to see that Ervin Somogyi does this as well. I don't believe it affects the sound of the guitar, certainly an arguable point, but I think any trade-off is justified by a large reduction in how fast environmental humidity changes get into the wood fibers. I admit to not having done any scientific trials, and I'd certainly be glad to see any, or hear any other arguments about why we should not give the insides at least that wash coat of shellac or thinned epoxy.

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Gene

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There are reasons like repairability and tone distruction. Then there is the natural ageing factor.

One thing I've pondered is the interaction with a domed top... if you have finish on the outside only, then the inner surface will shrink faster, tending to maintain the dome shape as it shrinks. The extreme opposite would be finish only on the inside, in which case it would be trying to go concave by surface difference in addition to the effect of having braces glued to it.

The question is, would it crack sooner from the quickly contracting unfinished inner surface, or slowly contracting from both sides? I'd say probably finish inside and outside would survive longer. It will fully equalize to the environment after a while either way, and the finished inside ought to delay that for longer.

However, one silly thing I worry about with it is not being able to smell the wood as strongly But I bet it would still build up that yummy rosewood smell while sitting in the case, and may actually stay stronger longer because of not losing smell particles so quickly

As Mark says, the ability to glue cleats inside if it ever cracks is one advantage. I don't see tone being an issue with a light shellac coat. Weight is pretty negligible and I'd be impressed if you could hear the difference of surface texture compared to bare wood

So the cleat issue and tradition are the only major arguments against it. Longer time before cracking from low humidity is the main advantage. I haven't made up my mind yet whether to do it on my next one or not. Just about ready to start the box closing sequence, so we'll find out which way I go soon

I think the repairability issue is somewhat valid, especially if epoxy was used. I will try some bonding tests looking at shellac washed vs raw wood surfaces. I don't expect to find a large difference. I'm not talking about a full finish coat like on the outside, just one or two light wash coats. Something to try next week after I get back from a trip I'm on.

I don't think that these light coats will cause "tone distruction", since they are much thinner than the outside finish coats. Thick coats inside or outside can change the sound in a usually negative fashion.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

Who knows - the opposite could be ture.
I shellac the inside of all my guitars - just one coat.

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We had this discussion on another forum and I don't believe anyone changed their minds.

I am a finisher of insides and will continue to do so with 2 to 3 coats of shellac. In my research on the topic for the last thread, I found that shellac is THE BEST vapor barrier finish we have and very effective.

One time after installing the rosette on a bandura top, I put a wash coat of shellac on the top to keep it clean. The top then began to potato chip for several days until I put a coat on the inside too. It then naturally flattened itself out after a day or two. I scraped for bracing and then re-sealed it.

I did not do this with a guitar and a bandura afterward and at least with the bandura that I took to a high humidity environment, the sealed interior instruments didn't move as much or at least moved in a more coordinated fashion than the instruments with sealed interiors.

Sealing the interior doesn't stop the MC of the wood from changing but it can slow it down and at least make it even on both sides of the instrument.

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Andy, I coat the inside of my instruments as well. That said I'm confused by this story. You seem to imply that coating the inside of a potato chipped top helped it. I don't see the correlation you are drawing? Obviously if shellac is a barrier, that works both ways.

Filippo[/quote]

I speculate that with one side shellacked and the other clean, one side was picking up moisture faster than the other causing it to pringle. I think that given enough time in a stable environment it would have flattened on its own. I further speculate that once I put the shellac on the other side, it slowed vapor transfer enough that the piece could achieve equilibrium within itself. Sound reasonable?

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Heh, missed this from a couple years ago. I think to the contrary actually. With both sides sealed, the moisture that was in the would could have a better chance of distributing itself equally through the wood.

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I add a few coats of shellac too. Shellac is definitely a moisture exchange retarder and that is good enough for me. Cracks don't occur solely from losing too much water, but from losing it too fast as well.

Bad for the tone? Seriously?

Recently I kept an African ebony headplate and a Macassar ebony headplate in a high humidity box in order to compare their expansion. Then I brought them back in my dry shop. The black ebony cracked. Some woods do not cope well with fast drying even if they are thin and totally free to shrink. If this was a back in a guitar it would cracked even worse. Not just black ebony, but I am sure that some of the more brittle rosewoods are very prone to this as well. I feel that headplate with a single coat of shellac it wouldn't have cracked. But this can be easily tested. Take two identical pieces of wood, same board, keep them in a wet box for a few days, bring them back in the shop, weight them and coat one with a bit of shellac. Weight them again after a few hours and the next day.

As for repairs, it is very easy to sand off a seal coat of shellac. You do need to sand the area anyway, as oxidized wood does not bond well at all.

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Build log

I have to agree , while I am no scientist I cannot see that the Tone would be affected by a coat of shellac . If this were indeed an issue then the best sounding guitars would be completely unfinished and raw wood.

I seriously doubt that given a line of guitars with one having a wash coat of shellac on the interior that the BEST ears in the business could pick it out.

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Yeah one coat is not going to change the sound, especially if it's just a washcoat.
At least to 99% of the people... Oh-oh here comes the other 1% with their meters and sinewaves. Just kidding.
I have always stained and finished every square inch on the furniture I built (OCD ) so it has crossed my mind to finish the inside of a guitar. But if you are trying to reach equilibrium both surfaces need to to be sealed equally or you are still creating a differential. And even though it has made me go hmmmmm.... many times, as I stated in the other forum of similar discussion I always come back to the fact that guitars are alive and well without adhering to the many hard and true rules of woodworking. So if you feel better about throwing on a coat or two in the inside, go for it. That's the whole beauty of being creative.
Make it your own. Or as the late Bob Ross used to say " It's your world, put a nice little tree there."

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It sounds to me like you don't have very good humidity control in your shop.

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John

That's absolutely true, at the time I did not have good humidity control in my shop and if I recall, it was end of winter or spring when we get a humidity swing around here. Since then I've gotten that under control. But the point still stands.

In that case, I had a free plate that was shellaced on one side and not the other. Humidity changed in my shop and the plate pringled on me. That same thing is happening to free range instruments out in the field, it's probably just that we're not noticing the effects or maybe we are but are attributing them to other causes.

Another anecdote - In a weird bit of irony, there is a Bandura Camp for kids and adults in Pennsylvania each summer at a camp ground on the Allegheny river valley. I say it's ironic because often, the humidity is like 90%+ for days or even a whole week at a time. Beautiful campground but it just kills instruments.

I had just finished an instrument and although I shouldn't, I take my new ones there because the bandura community in North America is so small and each year the who's who show up at that camp. I tend to thin my backs quite a bit and one year the humidity was particularly bad and my brand new instrument with an unfinished inside got just a nasty wave in the back. The top now looks about 40 years old with that telegraphing grain look to it. (The wave mostly disappears when it dries out)

The next year, I made another instrument with an even wider back, from the same plank as the last instrument but this time I did seal the inside. The humidity wasn't as bad that year but while the wave returned on the older instrument, the new one held up pretty darn well.

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