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On another forum someone mentioned flattening a top by humidifying with a wet sponge in a cup over a six week period. This doesn't make sense to me, I would think moisture would increase the bulge. Can someone explain this to me please?

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Hutch

Get the heck off the couch and go build a guitar!!!!

I'd like to hear an explanation for that too.
I must own up to having the odd brain fart myself...

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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.

it was on the net so it had to be true.
At least you can see that it was poppycock. IF the top is braced you want to see the RH of the area and see if the top is dry or that the top may have been braced with the RH out of whack. I brace my tops with the RH 40 to 45 % I figure I will be adding moisture from the glue , so I run the dehumidifier the night before to 40% and than has helped my tops to be more stable.

If your top was sunken because of RH you use the moisture to reestablish proper RH then maintain it. Someone posted something that they didn't understand. If you have wood that is cupping , that is a sign the rh is changing or that the plate didn't stabilize yet . I keep flipping them cup side down as the surface cells dry the shrink and pull the top cup up , so flip and this helps them to equalize. It may take a month or so form them to become stable. I store my wood for 2 years before I use it.

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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

Add to this that these can be structural implications - including a top with a lot of runout, some ladder bracing, and poor bracing, or bracing unglued. If one is removing the back, a potato chip top can be reworked - remove the bridge, remove the top bracing and bridge plate, place in a go bar deck with the correct radius, wet the top from the inside, and use go bars to press the top into the radius dish. Let dry for a few days. Then brace everything back up.

With RH changes some top wood thinks it is a potato chip, others not so much. To minimize the effect of RH swings you can do several things.

Use well-seasoned wood. If the top is not torrified, I like to cook it in the oven a week so before the braces go on.
Use well-seasoned brace stock and shape them during low humidity periods
Keep the RH stable, and low, during the bracing process.
Use CF in the brace material to improved resistance to top changes from RH changes if you expect the guitar is going to see wide
ranging RH. (Not for everyone)

I have used moisture and heavy, long term clamping to try and straighten a back but it was never successful long term.

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Joe Beaver
Maker of Sawdust

Many moons ago Taylor made some videos about humidifying guitars. In that video he states that the top geometry will change when the guitar goes from a very dry state to a humidified one. The AGF (I'm sort of assuming that's where you saw this?) Has been telling people what you wrote ever since.

The wood will move when humidified but they alwaya get it wrong. It does not do what they say it does.

We had a client denied warranty service on his high-end guitar because he listened to some BS on the Internet AND acted on it. We also turned him away because undoing the carnage was not profitable for us AND we are far too busy. The guy lied to me too and then admitted he lied so the hell with him, leave.

What I wanted to add is that regardless of if any level or RH manipulation makes any difference if that top was not built with 40 - 50% RH, seasoned woods, etc. it's a moving target for many years if not forever and likely not worth repairing in favor of replacing it with a stable top produced in proper RH.

It will also crack until the cows come home in time if you do get it back to shape if again it was not built in proper RH.

I always tell folks that job one of guitar building is working out what it's going to take to have that 40 - 50% RH range in one's shop for the duration of the building. Folks don't like to hear this they want to run machines and use edge tools, understandably but there is a reason why operating rooms are very clean.... it will bite you later if it wasn't. Guitars are no different since wood moves far more than... well never mind I'm a nice guy.

Hope you are doing great Sarge!!!

Just want to clarify something ...


It seems like most responses are with the idea of a top which is braced, presumably under the wrong conditions. Is the actual question about a top plates, a joined top or a braced top? I’ve seen all kinds of info and suggestions here and else where about what to try when you have a set of plates that have gone wonky.

I want to make sure we’re all talking about the same thing.


Sent from my iPhone using Tapatalk Pro

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If a top is flat enough to join without giving you grief and changes on you a few days after joining, (it may take a day or so for the water based glue used for the joint to lose the extra moisture) then I would suspect RH conditions in your shop have changed. If you joined at the 40-48% range and it is now 10% or more higher or lower, than it was during joining, I would wait for the RH to return to what it was. The top should flatten out, then you can shape the braces and glue. The brace stock should be kept in the same area as the plates by the way. Once braced all should be well.

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Joe Beaver
Maker of Sawdust