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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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"Indecision is the key to flexibility" .... Bumper sticker

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

I have a guitar from 1857 and the top isn't caved in. It isn't finished on the inside and had strings on it most of its life. I don't believe shellac or any finish inside a guitar is going to make it better than a non finished. There is way to much pseudo science on the web. Wood isn't that plastic. I agree that glue can allow creep but there are glues that do not creep.
A RH cycle isn't harming anything . It is duration of load that can go more in causing creep that RH cycles. According to your theory the top can cave in just from RH cycles , that ain't gonna happen. Bracing helps the structure to withstand the stresses and matching the bracing to the tops deflection ( young's modulus of elasticity) you can use science as a tool to build a guitar.
Building a guitar is matching art ( looking good ) and science ( design to withstand the stresses applied ).

The paper quoted was not about guitars it was about beams under load that is a different matter. 10 MPA is equal to 1450.377 PSI , . You cannot look at a beam load as the same as a guitar load. A top is braced the beam is not. Different structure and load sharing. Also wood can take more load in tension than compressive load.
Wood is a fascinating material .

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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 finish mine just like the outside of the box. Pore fill with epoxy, Nitro, finish sand to 200,000 grit then buff Looks really pretty and all shined up.

Might be some problem if repair is needed. But then I figure none of mine will never, ever, never need repair. And for sure should anyone pour water into it this will keep it safe, because I add coat of waterproofing stuff from a tv commercial that will let allow a boat with no bottom but a screen door float when added.

And for all of That humidity everyone is concerned about, doing this finish, I don't have to put in my instructions to buyers about how to care for the guitar in that regard. You know the normal care crap that one should be aware of with humidity when buying a $3000 + guitar.

Way I look at it, why not take care of all these concerns and worry about the waterlogging of the inside of a guitar from the start.

I have seen inside finished guitars cracked also so no if its gonna crack it is gonna crack

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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

When are we going to get a "Like" button on this forum???

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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."

Up until my latest instrument, I had never done any finishing inside an instrument. This time however, at the request of the customer, I have spray painted the interior flat black. I was pretty hesitant at first, but given the nature of the instrument, it worked out pretty well, and wasn't done on the top so I doubt it will affect the tone much.

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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."

I dunno, John...isn't that kinda like saying that brakes provide no protection against accidents, and if the car is gonna crash it's gonna crash?

Don Like button pushed for like button. Hell I wouldn't mind an unlike button at times. But very rarely here anymore so my like or unlike along with comments should be taken with the grain of salt

Most ascetics would prefer no finish on the inside, IMO ...

I certainly agree there's a lot of pseudo-science on the web. And that wood is a fascinating material! Wood creep due to load and varying humidity is actually established science. It's called "mechano-sorptive creep" -- http://scholar.google.com/scholar?q=mec ... creep+wood
That first pdf is published in a scientific journal (http://www.springerlink.com/content/l5525m28679r0524/) -- I posted the pdf because it's the whole article.
I'm confused about "wood is not that plastic", since I thought the (often-collapsing) 1930's Martins used hide glue. RH cycles alone (without load) would not cause collapse -- mechano-sorptive creep needs both.

Thanks for the conversion to psi... those are my usual units, too. Mechano-sorptive creep is a material property, so it can occur in any wood product (paper, bridges, guitars...). Some interesting snippets of info from 2 other articles:
"The mechano-sorptive effect is stronger for heavily loaded wood than that for lightly loaded. It is stronger for wood with lower elastic modulus and vice versa." -- http://www.springerlink.com/content/f219758768x25324/
"...whilst normal creep, under constant moisture conditions, depends on the time variable, mechano-sorptive creep does not..., depending only on the amount and direction of moisture change." -- http://resources.metapress.com/pdf-prev ... ze=largest

As good guitars are usually built, as Todd said, just stiff and strong enough, their peak stresses can be quite high. The max bending stress in the T-section formed by the X-brace + top is in the ballpark of 2 kpsi (1/4 x 9/16" rectangular-section Xs, 0.110" top... glad to post the spreadsheet). It's much higher for a triangular cross-section. It's probably also higher in the scallops, but I'd need FEA for an estimate. So with high stress, the wood is primed for accelerated creep if the RH were to cycle heavily. Of course, most guitars don't see a lot of RH swings, and for those I agree it wouldn't be a major issue.

I'm certainly not saying we need to design guitars like bridges or to last forever. Rather, that:
- If a customer wants to move their lightly-built guitar in and out of doors often, or can't control its humidity, shellac on both sides of the top would help it last longer (and also stay in tune). For the long-term, the whole inside need not have shellac -- mostly the highly stressed surfaces and braces. Just doing the X-braces near the bridge would probably help a lot.
- If you aren't sure what environment a guitar will be subjected to, and you'd like the top to last longer rather than shorter, you can probably help that happen with shellac. I'm not meaning to convince anyone to use it; just presenting info and options.

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David Malicky

You are way over thinking this a guitar is not under the same load as a beam structure. I have built for 14 yrs . Creep is not that big of an issue on a guitar of than the glue. As for finish inside a guitar I stand that it is a personal choice but there is not real need for it other than for personal choice, If you think it is then do it you certainly do not need my permission or endorsement.
I still stand that there is no true engineering or scientific study to say it is an improvement and in many cases facts never change a persons opinion . I believe in natural selection of process and if was as good as you say it is and was a fact more people would be going it. I see many guitars for repairs and had a Newell from the 1890's with a finished inside and it had more top and side cracks than my 1857 unfinished martin .

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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 readily admit I've only been building guitars for 5 years, so have much to learn. And I'd be happy if I were overthinking this. But if wood creep weren't a big issue, I don't understand a few things...
- how collapse could have happened to those vintage hide-glued tops.
- why many luthiers say a lightly built top (even with hide glue) will eventually collapse, and that's just the way it is.
- why a thin top eventually shows the braces telegraphing through.

On beams, the allowable stresses in those are actually similar or lower than for the X-braces in guitars. For the select structural grade spruces, the allowable bending stress (Fb) ranges from 1200 to 2200 psi (pages 5, 9, 12... of http://www.wclib.org/pdfs/AP-1.pdf)

The max stress in a triangular cross-section X-brace is more in the ballpark of a spruce aircraft. I'm no expert on these, but I gather they're built to a safety factor of 1.5 and about 4000 psi for bending. But they only have to sustain that stress for about 3 seconds at at time... perhaps a few cumulative hours over their lifetime? I'm sure Todd knows much more than I about that.

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David Malicky

A guitar is a truss structure , it relies on all the parts to help take a load and control stresses. If the tops shows the braces it was made too thin or the braces have become loose. Again it depends on what "brace shadow" you refer to. If under a load you can see the braces on the top, it is not that uncommon , if they are pronounced that is different . Lets look at what is happening on a guitar
The neck is connected to the body by the neck block , the strings are connected to the headstock and the bridge plate. This causes a rotational stress at the neck block from the string load. The Ball ends of the strings are being pulled up and towards the top of the saddle , This also causes a rotational force and a compressive load between the neck block and the bridge. These forces are coupled by the ball ends pulling up and the saddle pushing down.
Behind the bridge is a tensional load from the string pull , The RH creep has little if anything to do with the guitar, It is the stress applied by the strings that with time will do the most damage. RH shock can cause a cross grain stress that can dry a guitar and pull against the braces , this will often make the top sink and if allowed to go uncontrolled will eventually crack at a point of weakness. Or if the top is taking on more moisture cause the top to push on the bracing and cause the top to rise. As the neck block rotates, the fretboard extension is also pushing down on the top . This is often carried by the transverse brace. The neck rotation can also create a shear force that can cause a crack failure along the sides of the fretboard. Also the difference of expansion of the fretboard to the top can cause a similar crack. This can make the top appear to be caving in at the sound hole , also the rotational forces from the bridge / string connection also may help to push the top in if the brace structure can't control the load.
If the back is flat and the sides not contoured to a radius the back can flatten out allowing more rotation so you can see this is normal loading causing the body to change and nothing to do with RH creep. That is something all together different and is more for engineers to figure out a wood beam and loading in outside structure.
There are things that you can do structure wise to help but finish is a surface protectant . It will help if you drop water on it but not if you live in a wet environment , eventuality the wood will come to equilibrium of the environment it is kept. Then the glues can also move. Tite bond is more noted for creep that Hot Hide or Fish glue.
Hope this helps

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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 never gave that a thought Todd but I think to that I can agree. I had to look this up and do some studying.

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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

Thanks for the explanations, I see most of it the same way, and didn't know about the shear cracks on the sides of the fretboard -- that's very interesting. From what I've gathered, there's not enough clear evidence either way for whether mechano-sorptive creep can be an issue for guitars with uncontrolled RH. You may indeed be correct to say it isn't, and I'm not saying you're wrong, but as one of my old bosses would say, "In God we trust -- everyone else bring data." It's not a very hard experiment to run, but it could potentially trash some tops.

Many of us, myself included, have done a related 'experiment' -- leave a braced domed top in uncontrolled RH for a ~month. The dome is often lost when returning to proper RH. Whereas I've kept braced domed tops and backs in controlled RH for ~6 months and they've been fine. (Those stresses are fairly low, but they act on the top's weaker radial direction, so maybe enough.) But I'd want to run a controlled experiment to say what kind of creep actually happened.

On finishes, I completely agree that long term RH swings will go through any finish.

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David Malicky

the one thing missing from this experiment is the load of the strings . There are many physical things going on and when you stress a top from RH with the string stresses you may see this but often the string load will bring the top back up. There are physics involved with the geometry and environmental variables also.
Finishing a guitar inside isn't going to change this to any degree. Finish is a protectant for the wood surface.

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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

Mission Accomplished. Think of finishing the interior as a social movement to improve the lifestyles of repair men.

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