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I know shellac is a pretty darn good moisture barrier (as finishes go) but I wonder how much real effect it has on wood at soundboard thickness. Just how long does it take for such thin spruce to equilibrate? A good experiment would be to make three strips of soundboard thickness spruce anchored on only one end. One with no finish, one with full finish on one side and one with full finish on one side and a wash coat of shellac on the other. Then take all three through humidity shifts you expect a guitar to be exposed to. My guess is that the unfinished one will stabilize the fastest and the finished on one side to move the most, but where would the wash coated sample fit on the spectrum?

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

By the way, from the report I posted:

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Boat builder's have a different take on this I believe - they leave the inside unfinished because if it were sealed, it could result in accelerated growth of anaerobic bacteria which area actually worse than aerobic bacteria in terms of how they hasten the decay of the wood.

So, there could be a functional reason to leave it unfinished as well.

Trev

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Andy - thanks very much for posting that study!

I don't have any experience with finishing the inside of a guitar, but intuitively it would seem that a light coat of shellac or other finish would significantly slow down moisture absorption, which could help prevent cracking. I had a spruce top crack recently during a short but significant drop in humidity, and had to remove and replace the top. What a pain!


Some boat builders do "finish" the insides as well by saturating them with epoxy (WEST = Wood Epoxy Saturation Technique) which effectively blocks moisture from entering the wood, unless the barrier gets damaged. Also, in the case of boats, wood rot is a much bigger concern because of constant exposure to water, which a guitar will *hopefully* never experience.

I really don't think that wood rot is something luthiers need to worry about, unless you store your guitar in the hold.

Andy,
Thanks for posting this good resource (USDA Study).

Turns out that shellac, according to this detailed study, is better than nitrocellulose lacquer at retarding the flow of moisture through it into and out of the wood that it is protecting. That is an eye opener. (Finished/dried shellac hydroscopic? No way.)

I am surprised at some herein talking about not "allowing water to pass". Of course shellac and most all finishes will allow water moisture to pass. The issue is how fast it passes (the rate) and not that it does indeed pass it. Retarding/slowing the rate of water passage from the wood is an advantage. I can't see how there could be "no advantage" to this.

Changing the rate of moisture transfer from fast (no inside wood protection) to slow is indeed a measure of protection from cracking and from quickly occurring setup and tuning problems from wood movement due to moisture changes. From the study numbers it would appear that the old problem of traveling with a guitar from a high RH area to a very low RH area and back again in a few days would be largely aided or solved by using shellac on the inside of the guitar.

Are there "cons" to applying shellac inside. More cost to do? Yes. More repair time? Some repairers seem to debate this one since cleaning/sanding is required regardless.

How do the "pros" stack up again the "cons"? Of course everyone has to decide that for themselves. But I do not see the "pros" list being blank with "no advantages".

Good discussion.

1) Applying shellac to the inside hardly takes any time. Brush it on, wipe the excess down with a paper towel, and it might take two minutes to do the entire inside if you are really slow.

2) It takes at least two coats to give a high level of resistance to moisture transfer. But each coat goes quick, and you don't have to wait long between coats.

3) Keep in mind that all shellac (or any finish) can do is slow down the moisture exchange. It will not prevent it. If a guitar is exposed to an extremely dry climate for a few hours, coating the inside will make a huge difference. If the guitar is exposed for a few days or weeks, then neither shellac nor any other finish is going to keep the wood from drying out. Which means you need to ask yourself, "What is the situation I am trying to protect against?" I have never seen a situation in which a guitar was in an extremely dry environment for just one day. In theory, it can happen, but that's a pretty remote risk. In reality, the dry situations last long enough that applying a finish is not going to solve the problem. The solution is to build the guitar so that it can accommodate humidity changes, and warn the owner that he or she needs to keep the guitar in an appropriately humidified environment.

I'm going to chime in one more time. Up to this point, my instruments are UNfinished on the inside, but that might change on the next one. Regarding moisture movement in and out of the wood, I agree that nothing completely stops this phenomenon. But I do believe that a sealer coat of shellac on the inside and outside would tend to equalize the rate of movement through both surfaces, and that OUGHT to be easier on the instrument. As to wooden boats, I have never seen one that wasn't finished on the inside as well as the outside, but I admit I don't see as many of them as a coastal resident would. Regarding future repairs, I still think it's a toss-up. Seems to me a thin coat of shellac would actually make it easier to clean off grime, cobwebs--cigarette smoke (in the case of a weekly gigging guitar), airborne kitchen grease (see gigging guitar), etc. etc. etc. But again, I'll be the first to admit I've never done an inside repair on one of my instruments, so......
Patrick

Andy's reference doesn't specify whether their shellac is waxed or dewaxed, so I think the default assumption is that it's waxed. We know wax is an extremely effective moisture barrier. So I think that the high effectiveness of shellac is mainly because of its wax content. That could be a correlation without causation, but we also have no data on dewaxed shellac that would give confidence in it. I actually hope I'm wrong, as the wax content would make repairs more difficult.

BTW, the data from Andy's ref is summarized on page 12 of this pdf:
http://www.taunton.com/promotions/excer ... ngWood.pdf

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David Malicky

I'm not so sure about that. By the time shellac is used (i.e. after it's been sitting in a jar a while) it's mostly dewaxed anyway and, they talk about "white shellac" which could be what we call blond shellac - which is always dewaxed.

p.s. As a general principal, I don't worry too much about how exactly repairmen will curse my name in the future. I try and not do anything too stooopid but over all, I think it would be taking money out of their pockets to make life too easy for them.

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When you look at the scientific data on shellac , it is not a moisture barrier. I agree that any finish will slow down moisture exchange but it won't stop it. Shellac will absorb water weather it is waxed or not. It will make repairs inside the guitar difficult but it isn't insurmountable. The only benefit is the one you feel it gives but from an engineering point of view it is of no true value other than aesthetics .

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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

John,
I find your comments confusing. It appears extreme to say "water proof" and "barrier" and the implications you appear to imply. Most of the folks here are not talking about 100% stoppage of water/moisture transfer. They are talking about the RATE of water transfer, and that slowing that RATE down is an advantage. I would hope you would see that slowing down the rate of water/moisture is as a real advantage, yet you have said there is "no true advantage other than aesthetics" to interior shellac coating.

Nitro lacquer is also not "water proof" and not a "barrier" in the way you are using those words. Neither is poly. The USDA Study clearly points out that, with equal coatings, shellac slows down the transfer rate of moisture BETTER THAN NITRO AND POLY.

Does shellac absorb moisture? Yes, it does and it transfers it to and from the wood depending on RH. Does nitro and poly absorb moisture? Yes, it does and it transfers it to and from the wood depending on RH. Of the coatings/finishes, which has the slowest rate of transfer? Surprisingly, shellac is better per the study. It seems pointless to me to talk about 100% stoppage of moisture transfer since there is not a practical coating for guitars that does that. So I am confused as why you say “it won't stop it”.

I am not trying to convince you to use an interior coating. You have decided not to. That is absolutely fine. But I am wondering how you have come to the conclusion that slowing the rate of moisture transfer via an interior coating is of no value or advantage? (or, logically, an exterior coating for that matter).

This is not an attack at all. You are very knowledgeable. I am just confused by your comments.

Ed

Shellac or Nitro is not truly water proof but both inhibit water absorption. From a repair point finish inside a guitar can make some repairs more difficult. Adding finish inside a guitar is a personal choice and I know many classical guys like to do it but in all honesty , it won't make for a better or more stable instrument. It is your choice for ascetics .
I don't know many steel string guys that do it . Somogi does but Henderson , Nichols , Myself don't and we just don't feel it is needed. I like that the wood can breath and with open wood inside the wood can acclimate well . Adding finish inside also adds mass but I doubt that it will be detrimental .
If you like it and feel happy with it use it. I just don't see any need for it. I think this is a personal choice . Finish is there to protect the wood surface from wear . Too much finish can take away from tone if the top get over coated.

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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

I think a common cause of cracks is a scenario in which adjacent portions of the same piece of wood contain different levels of moisture, resulting in tension because the portion containing more moisture expands, placing pressure on the drier wood, resulting in a crack on that “side”. The crack may ultimately extend through the piece. A coating of shellac (or other) might slow down the rate of moisture exchange, allowing the wood to absorb or release moisture more evenly, mitigating the tension.

That’s how I see it anyway.

In a recent thread about acclimating newly acquired wood, it was suggested that one way to determine whether wood is at equilibrium moisture content is to lay the pieces on a flat surface and observe how they behave. If they stay flat, they are at equilibrium. If they become concave they are releasing moisture and if they go convex they are absorbing moisture. I’ve done this and the movement happens pretty rapidly.

Now imagine the same thing happening with a finished guitar in which the plates are finished on one side and unfinished on the other, and the plates are constrained by radii and bracing. If the humidity change is sudden enough, how can it not create tension in the wood, especially in relatively unconstrained areas like the lower bout area of the top? In the case of a rapid drop in humidity, I would think any weak areas would be susceptible to developing a crack on the inside of the top, which would eventually work its way through to the outer surface.

FWIW – I mentioned earlier in the thread that I had to retop a guitar recently that developed a crack in the lower bout. The guitar was unfinished but did have a light protective coating (3 coats IIRC) of shellac on the outside.

Just sayin’.

Interesting! I looked up white shellac and found a little info... On the dewaxed side, the only seller I found has it dewaxed and Frank Ford uses "white" and "blond" synonymously:
http://www.ubeaut.com.au/dewaxed.html
http://www.frets.com/FretsPages/Luthier ... ellac.html

but apparently a waxy white shellac has been sold in paint stores, and the definition is simply that it's been bleached:
http://www.woodworkforums.com/f9/best-w ... er-148002/
http://www.frets.com/FretsPages/Luthier ... ellac.html
http://www.merriam-webster.com/dictiona ... %20shellac

On the wax settling out, I guess it depends on whether the FPL folks shook the jar or not.

Nothing like ambiguity and equivocal evidence to clear things up. Anyone want to run an experiment?

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David Malicky

Prior to this thread, I would not have bothered to coat the inside of a guitar. While I admit that, to my eye, the coated looks nicer than uncoated, the look is not enough reason for me to coat it and perhaps make repairs more troublesome.

But slowing the RATE of moisture change is a much more important advantage to me. I think there are several "results" along the way to the "end". I see each result as a separate advantage Here is an example of what I'm thinking:

Let's assume that a guitar is in a constant 45% RH environment inside a wooden case all of which is in equilibrium. Then each of following situations occurs:

Situation 1 - Very Short Term Tuning Advantage:
Assume the guitar is pulled out of the case in a low 20% RH room for playing a couple of hours and then returned to the case and the 45% RH room.

The advantage here is tuning for the interior coated guitar. The slower RATE of its giving up moisture makes the guitar much more stable during this 2 hours window of playing and there are little or no problems with keeping in tune. Here is WilliamS’s testimony of his actual experience with this issue with his uncoated and coated guitars.

[quote="WilliamS"]If, for example, I take my 40 year old Martin 00-28 nylon string (my first guitar-lucky me) up to the top floor of my house right now, which isn't humidity controlled and play for an hour I'd have to retune about three times to compensate for the guitar's movement from the current extremely low humidity. Roughly the same is true for the other guitars I have with no sealer inside. This is not at all true for my guitars that have a wash coat inside. [quote]

Situation 2 – Medium Short Term Maintenance Advantage:
Assume a professional musician flies from one coast to another on a 2 to 3 days gigging tour during the winter.

In this situation it is not unusual for musicians to report that their guitar, which did not buzz at home, is now buzzing to the point that it needs some set-up adjustments. A slower RATE of moisture transfer could, and likely would, stop this problem from happening. The USDA Study shows that uncoated wood changes very greatly in just one day. But the shellac coated wood takes 7 to 14 days to be similarly affected. So the interior coated guitar will be more stable during this short gigging duration requiring less maintenance.

Situation 3 – Long Term

Assume the guitar is moved to 20% RH for 6 months.

I think this is the situation that Filippo is talking about. In this case, the coated guitar would end up at the same equilibrium as the uncoated guitar. Would there be a difference in cracking? I don’t think we can say one way or another. There may or may not be an advantage to the interior coated guitar. According to the USDA Study, it will take the coated guitar several weeks longer to reach equilibrium with the new environment which I think is good. But regarding cracking there may not be an “end result” difference.

In my mind, Situation 1 and Situation 2 are important advantages, more so than aesthetics. Because I do not know the answer to Situation 3 does not mean I want to give up the advantages of the first two.
This thread has been helpful for me to think through the pros and cons. I think I’ll change to shellac coating the inside.

Ed

The only crack I had so far was on a Madagascar rosewood back which was well coated with shellac on the inside. The owner borrowed the guitar to a friend who lives in a very small room, very hot during the winter, which happened to be quite severe here this year. To make matters worse he would open the window daily to cool the room so I imagine the humidity was dropping from 30 to 20% at times. After 1 month the frets were sticking out and the back cracked from next to the neck block down to the waist. Another 3 very small cracks appeared here and there on the sides. Fortunately the spruce soundboard did not crack but it was looking a bit crumpled around the braces, same for the lower bout of the back. It was built at 45%.

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Build log

I think (but what do I know) that the advantage would be in slowing moisture exchange, essentially buffering the movement in sudden extreme conditions. Clearly, with long enough exposure the wood will reach equilibrium no matter what is or isn’t on the surface. My question is if the effect is enough to bother with. I’m not sure how long it will take 0.1 inch spruce to reach equilibrium. This will certainly happen much more quickly than it would on cabinet thickness wood. I’m sure there is an effect, just not sure how relevant it is.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Tim McKnight finishes the insides of his guitars and believes it slows down moisture exchange significantly. Here's a quote from him from the AGF thread linked below:


http://www.acousticguitarforum.com/foru ... 679&page=4

After reading the USDA Study, Tim McKnight's testimony does not surprise me as being true. Its the Situation 1 and 2 that I was describing above.

I've been building custom cabinetry for over 20 years, and I have always finished all sides of cabinetry and table tops, and have not had any issues with cracked or warped wood. This is not just for aesthetics. To keep solid wood flat and crack-free, moisture needs to migrate at a more or less equal rate from both sides.
The radiused braces will maintain a guitars shape under normal conditions, but when you go to an extreme condition problems occur.
If both sides are finished, extreme conditions will still affect wood, but not nearly as fast. Try putting one thin coat of any finish - danish oil, poly, shellac, lacquer, even glue, on one side of any piece of wood, stand it so that both sides are exposed to the air, and see what happens. If you are in a hurry, mist both sides with water.
I am not saying one way or another that finishing the inside of a guitar is right or wrong, just that wood breathes, and behaves better when breathing from both sides.

Alex

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the point is that no one has a scientific proof that it does anything other than make it look nice. As for stress on a joint from moisture , the wood will constantly be in flux and that is part of the design . Braces will come apart usually from age or severe stress. Also if you build in a stress riser , that may also make a glue joint fail and is one reason we fade braces that are not supported by the kerfing.
The science is that shellac will still allow moisture to penetrate the finish but I agree would slow things down if it were exposed to a sever or sudden RH change. There are many points against doing but if you like it by all means do so as it won't harm anything.

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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

Ok so like many topics in OLF we have concluded that .... Some do and some don't.



Mean while no one's guitar has blown up.

Another issue is the long-term creep of wood under humidity cycles and constant load. Wood creeps on its own, of course, but it creeps markedly faster when humidity is cycled. See Figures 2 and 3 of this study for some interesting experimental data:
http://www.innventia.com/Documents/Rapp ... t%2082.pdf

The paper below found that there is no limit to the extent of creep as humidity is cycled: it keeps going, and going...
http://www.springerlink.com/content/r74r328356145x5m/

So, every humidity cycle that a guitar experiences puts the top one step closer to caving in, etc. Most nice guitars won't experience a lot of severe cycles. But for a guitar moved in and out of doors often, shellac would cut down on the number and extent of the moisture cycles.

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David Malicky