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Certainly on classicals I've played, you deaden the sound if you hug the back. So the back is involved with more than shape.

I wonder if a light finish inside reduces dust adhesion, and maybe reduces the need for frequent interior cleaning?

Filippo Morelli got it right . I do understand the importance of the guitar and its weight . The top is the major contributor for tone. For the record I don't want to upset anyone , but I do understand the science behind the guitar . The back and sides do infer a bit on the guitar but it is the top and the air cavity that will do most of the work.
this is my last post on this subject . I agree that we are allowed to disagree , lets not get disagreeable . If anyone took any offense , that was not my intent , but in all forums when we are dealing with different theories and ideas we will at time not see things eye to eye . No true discovery was made by status quo thinking , and it is the dreamers that often make the discovery , still , not all ideas are good ideas .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Hi John - please don't take any offense if it seems as if anyone here is questioning you - questioning posts and information shared is the basic purpose of forums, discussion forums.

Personally I could care less if anyone uses shellac on the inside of their boxes. As stated previously I have not yet done this but I do see that there is some potential benefit in certain situations. For example one of my customers came over yesterday afternoon and his Heshtone went from 30ish RH in his house to 20ish RH outside where it sat in the back of a Ford Expedition while my client had a few... and then to the 45ish RH of my shop where we jammed and my client had a few more... I actually started wondering if it would be easier for this guy to use beer inside his Planet Waves humidifier.... But I digress....

I worry about guitars in this climate and even though I take great care to hammer the points of good RH management home to my clients people are going to be people and perhaps not use the cases with humidifiers, keep the guitar in the case, or use the Planet Wave humidifiers that I supply with my guitars. BTW as you mentioned keeping the guitar in the case in the winter is a great practice to do - highly recommended!

As far as engineering data to support that sealing the inside of the box has a benefit I would bounce the question back and ask for data that supports that sealing the inside of the box does not slow the potential harm of rapid RH changes.

I have played two guitars that were sealed on the inside with shellac and both guitars were 2 of the very best sounding guitars that I have ever played. Both were Somogyi's. I don't know John if Somogyi has built as many guitars as you, over 100 as you said and say frequently..., but I suspect that Somogyi is a competent builder.

Regarding mass being added by shellac and shellac being hygroscopic: My impression is that if one tested to determine how much mass is added by shellac they would find it to be, well... mouse nuts.... The alcohol in shellac will in time flash off and leave only the shellac in it's very thin layered form. The one entire guitar that I french polished had just under 2 ounces added to the entire guitar by the process of Z-Poxy pore filling and many, many coats of french polish. This leads me to believe that sealing the inside of a box with a coat or two of shellac is going to add grams and not ounces.... and again the weight of the alcohol will flash off. My impression is also that it is the alcohol that is water absorbent not the shellac or that shellac is minimally hygroscopic.

BTW I want to take this opportunity to wish everyone Happy Holidays!!!!!!

Bogdanovich does not seal the inside of the guitar, not in his book anyway. He shellaces (let's make it a verb…) the back braces and centre graft only, the back is left finely sanded or scraped, but raw. He does finish the inside of the sides, though.

I wonder if it's not more of a visual thing, like let's make the inside shinny, nice and tidy. I do not believe in the moisture exchange being slowed down by a washcoat on the inside.

I definitely do not shellac the inside of my guitars, and have no plan to. And I do not notice problems in keeping the guitars in tune in spite of the wide RH changes we have here in northern N-E. I keep my personal guitar in the house, outside of the case, wherever I fancy playing it, and do not pay attention about the RH, heat, cold or anything. Well, I do not keep it too close to the woodstove… It's meant to be played, it's a tool, and it seems to be doing just fine.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

I don't use any finish on the interior of my guitars.
However, I recently restored a 1959 Hernandez Y Aguado guitar that was finished on the inside, but only in the areas that you could readily see from the soundhole. The inside surface of the top was not finished, as well as the back below the third brace.
Thus I can only surmise that it was done to enhance the interior appearance only.

Same here, If I put it in the case I'd never get around to playing it.

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Say what you do, Do what you say.

This may sound a little naive but other than causing an issue with cleating a future crack how would it make interior repairs so any more difficult? Maybe since I've not had any real warranty repair issues I may be missing something that should be obvious. Since it is applied after all the braces are glued in any loose braces could be reglued with no added problems. I can only imagine you'all are talking about cleats. If that is it then that's not really a big deal. A little light sanding or scraping will quickly remove a very thin wash coat of shellac.

I've never personally done it but probably will really soon. I plan to apply it with a pad and just a couple of dilute coats would be all. And my sole purpose would be to help with setup stability and changes due to weather, temp. etc. At least here I see up to 2/64ths" increase/decrease in string height over the course of going from summer to winter...if not being diligent with my humidifying/dehumidifying.

SR

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"We might not be big but we sure are slow"

I've finished the interiors of some of my guitars with shellac(back & sides only), I don't do it now, but those that had the interior finished sounded just as good as those not receiving it.......relative to tone, I don't think it makes a lick of difference either way.

Greg

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Gwaltney Guitars

It shouldn't be too hard to devise a test to determine, with certainty, whether a wash coat or two of shellac actually make a measurable difference in moisture exchange. I don't know the answer to that question. Maybe such a test has already been done - anyone know? Shellac, in general, when applied as a complete finish, is proven to be quite effective at slowing moisture exchange. If a wash coat or two can be proven to significantly slow moisture exchange, then there is a clear advantage to applying it to the interiors of guitars, "pro" or "con" anecdotes notwithstanding.

Shellac hygroscopic? That's an interesting statement I've never heard. I'd like to know what data support that. (Shellac can certainly be damaged by direct contact with liquid water - it is one of the most vulnerable finishes in that regard - but that's not what we're talking about.)

Shellac can craze under certain conditions, especially if applied thickly, but there are many very old shellac finishes that have remained in remarkably pristine condition. It is not as inherently prone to deterioration as many other finishes.

I'm not sure about the claim that shellac yellows. Shellac, of course, often has natural color in it when it's applied, some types being quite strongly colored.

Shellac is actually pretty amazing stuff!

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Page 218 of Bogdonavich's book: "...When dry, remove the clamps and give the inside of the top a coat of shellac (11-27--this refers to the image of him applying shellac to the soundboard's interior). This will help reduce moisture exchange with the environment and add stability." His back is not sealed but his sides and top are.

Wood is hygroscopic too btw.

Shellac certainly does slow the "breathing" and it may be a good thing. I was once building an instrument and I put a wash coat of shellac on the outside side of the top before it was glued down or braced because I had already done some sanding and such and didn't want it to get dirty. I did not put a wash coat on the inside side. That top would potato chip practically from breathing on it until I put a wash coat on the other side. In the end I ended up sealing the entire inside of that instrument with I think two coats of 1lb-ish cut.

From an engineering perspective, it makes sense to want to have the top reach equilibrium with the air in a calm and measured fashion on both sides simultaneously. The example above shows the kinds of stresses that can build in a piece of wood when only one side is sealed.

Can you explain to me, from an engineering perspective, why it's better to have a top reach equilibrium more quickly but in a severely asymmetrical way compared to a slower but more uniform way?

p.s. other than that one instrument, I haven't been sealing the insides but I've been thinking of starting to, this debate is pushing me more in that direction.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

"hygroscopic" means "readily taking up and retaining moisture"

Catastrophic cracking of wood occurs when wood rapidly loses moisture, not when it gains moisture. So, it would seem to me that the hygroscopic quality of shellac is beneficial, not detrimental.

This discussion isn't about boat paddles or bathtub toys, and the fact that shellac is not impermeable is moot. All that is being asked of this material is that it provide a buffer to slow down the rapid loss of moisture from the surface of the wood. Just a buffer. Guitars coated with shellac on the inside can certainly be made to crack with a rapid loss of moisture, but it should take quite a bit longer for enough water to move out to make it happen.

Every one of us has the materials to perform a test: two long crossgrain offcut scraps from the same piece of thinned (soundboard or back) wood. Check to make sure shop RH is around 45 to 50%, and then coat the samples with shellac - one on one side only, one on both sides. When thoroughly dry, hold both pieces by one end in one spring clamp with the other ends loose, subject both scrap pieces to a dry room test, and watch.

Testing for full-on catastrophic failure using a pair of completed instruments would be too heartbreaking, and (because the wood is glued as a box) would show cracking instead of just movement, but the scraps should be enough to illustrate the point.

Dennis

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Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

I have thought of doing this but never have because i had thoughts that it might effect tone.

This is the main reason i don't do it.

All very interesting, but until you do what this guy does, you don't know interior finishing:

Well dang, I don't know how to post the pic. In any case, google Taran Guitars, find his blog page, and scroll down until you see his FRENCH POLISHED GUITAR INTERIORS pics. Amazing.

I did attach one of the pics. You owe it to yourself to see the others. He calls himself 'anal' and who am I to disagree?

I will be a very very cold day you-know-where when I do anything even approaching that.

Other than that I do not have a dog in the fight..
Dave

Here's another angle of the FP interior.

Well Mark,

Me ain't built anything that has outlived me yet so me don't know about aging effects.
However, on occasion me has taken it into me head to pour a quart of this that or the other down the sound hole, (usually black paint) sloshed it around and poured it back out. It helps hide things.

Now it didn't have any effect what so ever at all on what ever me managed to pour back out, and well the inside looked really clean after it dried.

Been thinking of pouring in some of me 'om' brew beer and seeing if that helps the both of them age.

Then oft the thing to a bar musician who can appreciate the aroma.


blessings
the
Padma

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Audiences and dispensations on Thursdays ~ by appointment only.


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+1. That right there is enough for me. I guess I don't buy that there's any real downside to a washcoat or two. I just know I hate reaching inside a guitar and it feels all sandpapery and like dust is all catching on that roughness.

Peace,
Sanaka

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...imagine there were no hypothetical situations...

I wash coat the inside of the back and sides of guitars with shellac. It acts as a buffer against sharp humidity changes. It does not stop moisture and that is good because that would be a bad thing, but it certainly does slow the movement of moisture in and out of wood. Shellac does not affect the tone of the instrument at all if you do not coat the top. I have taken guitars and coated them on the inside after completing repair work because the customer was heading north and the shellac has made no noticeable difference to the tone or volume what so ever, not to me, and not to the owner of the instrument who had played it for years prior. You can assume a lot just thinking about it, but you really need to try stuff out before you can actually know. As for slowing down the aging process, well all i can say is that we are not talking about 'drying' the wood here cause that's already happened. That's why moisture is traveling in and out with changes in RH. I suspect there would be little difference in the long run in that department simply because i do not prescribe to the idea that the aging or playing in process or what ever else you call it is related to exposure to air, rather it is an internal thing and is going on at a cellular level.

Cheers

Kim

Exactly, in a repair situation you would need to freshen up the glue surfaces with abrasive prior to applying adhesive regardless of whether or not a wash coat of shellac had been applied during the build process. This is simply not an issue... but shellac will help when blasting all the dust and crap out of the guitar with compressed air before you start work.

To each his own for sure, but unless they are clearly stated as such we should try to leave our assumptions out of the conversation if that is all they are. Do the experiment Dennis has suggested, see for yourself and then have a think about what is happening.

Cheers

Kim