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Does anyone here seal the inside of the guitars they build before they glue them together? It seems to me that it would help mitigate the problem of humidity, and perhaps create a harder surface that would reflect the sound better, but I never see anyone do this. Is there reason that few people do this?

Most finishes don't provide that good of a barrier to moisture and make it more difficult to perform repairs. Removing the finish to provide a good gluing surface for cleats and such would be difficult working through the soundhole. Plus it would be more work and more (finish) material , both of which would add to the cost.

Nobody does that first of all... (well, I'm sure some do, but all that extra work for nothing?)

If you're going to paint the inside of the guitar, then find a way to make it look good. That's going to be hard because you can't sand and polish the inside of the guitar. So satin finish is your only option to make it look good.

As others have mentioned, gluing in cleats for crack repair is going to be impossible without a complete disassembly.

Also how are you going to make sure you don't end up gluing finished portion of the soundboard when closing the box? You might end up with areas that are not fully bonded and would open up on you in a few years.

If the customer won't keep it out of extremes of temperature and humidity, sealing the insides aren't going to make the guitar more durable all of a sudden.

The only finish that will possibly make a difference in terms of humidity difference is THICK layers of epoxy. Your guitar will sound worse than 50 dollar pawn shop specials if you do that.

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

I don't have your encyclopaedic knowledge of builders but I believe Greenfield Guitars finish at least some inside (see "Où se trouve" on youtube at 13:57 and 14:17)
Rory Dowling of Taran Guitars does as well, and I as I remember at least one member of OLF.
I'm sure they have their own reasons.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Those that do, how do they keep the inside of the guitar looking good when finished? In my experience when you put finish on wood, no matter what kind of finish it raises the grain, and you have to sand between coats to make it look better. But how do you sand in between braces? Also, what if cleats are needed for repairs, did they make the finish thin enough so that a repair man can remove the finish for repair?

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

Somogyi and many of his acolytes do that for OP's stated reasons. Not sure of its efficacy but they do it.

One of the pitfalls of being a newer builder can be ideas that we think have merit and are even original but when we instead do the diligence to research things we usually find that very little in this trade has not been considered or thought of before.

If you search the archives here this topic has been brought up countless times and here is a summary for you of the answers:

Sealing the insides reduces the serviceability of the instrument in respect to not interfering with glue sticking in the future should a brace come lose or a crack happen. Servicability is super important and speaks to the value that your wares actually deliver not now but over time.

Most interior attempts are finishing only belay moisture permeation through the finish (or around it...) for a few hours making it in my view a useless and even poor practice. It usually looks like **** too also IME.

It's more prevalent in the classical guitar world for some reason or another but still it interferes with the serviceability of the instrument. I don't have statistics but I doubt that more than a fraction of a percent of guitars have had this done and again it's because it's totally useless.

Sure the finish can be removed by the repair Luthiers like me and my shop but I'll charge you more and likely tell you that your builder should not have done that. Who needs to go there....

Welcome to Lutherie and I'll add that you have so very much to learn as to what we do do and why that my suggestion to you is to simply see your early guitars as woodworking projects with completion and learning all that you can from them being the goal not attempting to reinvent the wheel so to speak.

Happy building.

My Kohno 30 has a clear finish on the inside.

Serviceability is not something a lot of players think about when spending their hard earned money on a guitar. There are lots of fake Martins out there, uses the same material and all that, but sounds terrible and I'm sure its serviceability is basically nonexistent (as in don't even think about a neck reset, when it happens just throw the guitar away).

I went to a music store who are supposed to be a Martin distributor in Taiwan, and they have a bunch of Mexican Martins on display. None of the real Martins (D28, etc.) are on display, you have to ask for it because they're afraid of Taiwan humidity ruining them... Anyways, they got these guitars that looks very much like a Martin, until you look up close and you realize it does not say Martin on it (but it's placed really close to Martins). I picked it up, and first thing I noticed is the action is VERY high (possibly bellying because of humidity), and the sound is so-so, not much better than Mexican Martins. Then I asked for a real Martin OM, and the sound on that is amazing. Also I know Martins are serviceable... if not glued in dovetails it's bolt on.

I think the shop is really shady for putting fake Martins next to Mexican Martins... and also they should have a room that is humidity controlled for customers to go in to try the more expensive guitars... no music shops in Taiwan do this.

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

I suppose these would be a case of looking for the solutions rather than focussing on the problems.
As Hesh said, repairers will charge more (and possibly throw in a choice words)
Some of the makers mentioned in this thread doing this build excellent guitars, beautifully detailed, and have very long waiting lists at prices that make a custom shop Martin look relatively inexpensive.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

The reason for asking this question was because, a piece of wood, finished on one side only will warp, even if it is perfectly quarter-sawn. It will draw in water easier on one side than the other when it is humid and dry faster from that same side when it is dry. The constant back and forth warping is probably the reason you need to make as many repairs as you do. When a piece wood is restricted from warping, it will crack. We all know this. So to have the piece of wood sealed evenly on both sides, will not stop the moisture being drawn into the wood but it will be drawn in more evenly and that causes less stress on the wood

Shellac is the common finish used as a wash coat in the inside of the guitar. Even just painting on a coat it still looks good, if you really care about what the inside of the guitar looks like. It helps it stay clean too. I've never heard of anyone finishing the inside of the soundboard. IMHO it's a useless practice but at the same time I do question the serviceability argument. If a brace cracks then it's still wood on wood, if it cracks at the glue line then you have to deal with that for what ever glue was used, if anything the coat of shellac would make glue cleanup easier with a damp cloth. If the top cracks you can still cleat it since it's not finished anyway. So that leaves the backs and sides. Lots of repair techs don't use cleats anyway and in the case of a guitar finished on the inside you can just use CA to glue the cleats in.



Yup that's the common argument in favor of it. But there have been studies that show that basically the finishes we use, and more importantly the thickness, of the finishes that we use on the outside of guitars are practically worthless for moisture control. The guitar finish is pretty much there just to make the instrument look pretty and stay clean. Nitro for example (I don't have the exact numbers here) has to be something like 10 times the thicknesses that we use on guitars in order to start being a true vapor barrier. But as guitar makers seeking the best tone in our instruments we's never put it on that thick.

But still, you are right, in that a board will warp on one side as moisture enters or exits faster. So the coat of shellac will at least retard that a bit, a very very small bit. But the question is, does it really matter on a braced back attached to the rims of a guitar where moisture can only come in at a fast rate through a small sound hole?

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Like I said the only truly effective vapor barrier is a THICK coat of epoxy, as in at least 1mm thick, preferably reinforced with fiberglass. But if you put that much stuff on any instrument it's going to sound pretty bad.

_________________
Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

A quick experiment on two pieces of scrap spruce, one finished on one side only and the other on both sides, would be enough to show you how much the wood moves from one climate to another. It’s not insignificant.

You can see there is a divergence of opinion on the value of finish on the inside of the guitar, from luthiers at all levels of skill. As with many things you picks your poison and takes your chances.

Acoustic guitar makers routinely do things that would make other woodworkers cringe. We finish only one side of a panel. There are tons of cross grain glue joints. tops and backs are rigidly attached to a box that will move differently. Even jointing and gluing an edge with only around 0.1" of gluing thickness seems strange.

Flying in the face of these conventional wisdoms has some problems, some solutions and some reasons that it is okay in this application. Guitars sometimes need to be repaired or necks reset; we accept this because of the nature of the way they are constructed (example of a problem). Top and back plates are domed which allows the cross grain panels to move without tearing themselves apart (example of a solution). The wood is thin, very thin. Moisture fluctuations don't take nearly as long to propagate through a thin soundboard as they do through a 1 inch thick table top; coupled with the very thin finish, they equilibrate much faster to RH. Guitars are generally (or should be) taken better care of than furniture too (examples of reasons it is okay).

It is important to not dismiss conventional wisdom and there is nothing wrong with trying to apply information to this craft. To Hesh's point, there are may factors in this craft that are just different than other woodworking crafts and it takes some time to see that. A good way to look at it is, Lutherie has its conventional wisdom too; it may be different that other applications but there are usually reasons for it.

Guitars are built very lightly, subject to tremendous long term forces and asked to vibrate constantly. When you think about it, it is a wonder they hold together at all.

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Bryan Bear PMoMC

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

If I'm not quite senile yet, I seem to remember Art Overhotzer recommended Thompson's Water Seal in his book.
I guess if your guitar dosn't work out you can use it for a decoration on your deck.

Most of the salient points appear to have been made, but I will add a few:

- In my opinion (and only mine, should the boss or his minions be once again engaging in surveillance of my posts here), the worst reason to finish the interior is that it seems to clean up the interior with less work than the sort of progressive cleaning, sanding, and scraping done at the time braces and other components are fitted. This is not to be confused with the work done by Mr. Somogyi, his acolytes, or many classical builders, who do both that progressive cleaning and interior finishing.

- Regluing braces is not the concern - grafts are. We seen dozens of guitars per year with failed crack repairs...many of them without graft support or with clunky, chunky, two-generations-out-of-date ‘diamond’ cleats. These older style cleats should be an anathema to repair people. They are more laborious to install, concentrate - rather than distribute - stress, and look hideous in the same way that Lycra can when misused (seriously - spandex is a privilege, not a right, people). That these older- style grafts are awful is no reason to stop using grafts - instead, the failures seen in ungrafted cracks are reason to use low-profile, contoured, linear grafts where necessary (see Mr. TJ Thompson’s work for suitable detail). To borrow a famous meme, we don’t graft every crack, but when we do, we do it such that we a) restore cross-grain integrity, b) don’t create new problems, or c) add to the headaches of future repair people.

- The film thickness needed to get meaningful resistance to water vapor is significant...and most interior finishing we see does not come close to that thickness, let alone ‘balance’ water vapor exchange through the outer surfaces of the instrument.

- The metric of finish goodness for guitar builders is wrapped up with the notion of ‘the thinner finish is the better finish’ as far as acoustic performance is concerned, so doubling that thickness in the name of the dubious virtue of ‘balanced water vapor exchange’ seems like a poor trade.

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For the times they are a changin'

- Bob Dylan

Where would I be able to see Mr. Thompson's linear grafts?

https://proluthiertools.com/product/top ... ir-cleats/
https://proluthiertools.com/product/top ... corrector/
https://proluthiertools.com/product/bac ... epair-kit/

cheers

http://www.stewmac.com/How-To/Tool_Demo ... Video.html

I wonder what the grain orientation is of these "linear grafts" Using lexan and magnets is a pretty slick way to put them in place.
Diamond cleats if well done aren't particularly chunky or clunky and the grain can be oriented across the grain of the crack. Not to be a luddite, but diamond cleats do have a long history in the instrument repair trade. When properly made I don't see how they would cause a stress riser.

edit: looking at the pro tools link posted I can see the linear grafts do have the grain oriented to cross the grain of the crack.

I use diamond cleats, always have and always will. Sand them with tapered feathered edges to ween off the stress riser. They don't need to be massive, 1/16th inch tapered to zero is good enough. Magnets are truly wonderful aren't they. Some builders even use cleats on the top center seam during construction.

When you have a top crack that hasn't been repaired for a while and thus is packed with dirt, combined with old finish there's not much of a way to get a good bond. You'd have to knife it out and patch it, then cleat it. Even if you used sitka spruce to sitka spruce, old finish/wood is darkened so matching the color is very difficult.

I had a job like that once.. knifed the old wood out because it was packed with grime. I cleated it and then inserted some sitka spruce in it. It is still visible but structurally sound.

_________________
Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

We have used well tapered diamond cleats on short cracks that are fully covered by a single cleat, but a string of diamonds on a crack ends up concentrating cross-grain stress between diamonds. If well-tapered, the change in stress is more gradual than coarser grafts, but there are still a string of points along the crack which are unreinforced where diamonds meet.

The fix for this is to overlap the ends of the grafts...which when carried to the extreme look, look like a cross-grain strips. In other words, Mr. Thompson’s cleats are just the logical development of diamond-shaped grafts..the strips are faster to make than proper diamond grafts and easier to align for installation, so save time. We mill our own strips from spruce, redwood, cedar, mahogang, rosewood, etc., and are thinner and more aggressively tapered than what StewMac or Mr. Thompson sell. We don’t use the acrylic cauls for the job, but I’d like to try them at some point.

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For the times they are a changin'

- Bob Dylan

Yes, I put a light spray on the inside when I expect the guitar to live in a humid climate. It won't stop moisture absorption but it does slow it down. If the player brings the guitar out to the patio on warm southern night for an hour or so, it won't mean instant potato chipping with the wood and finish problems that follow. The repair guy may have to do a little scraping before cleating somewhere down the road but so be it.

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Joe Beaver
Maker of Sawdust