Guitar back plate. Problem 324.

[ClaptonWannabe2]

Well my back is glued and braced. Cut to fit. But the problem that occurred. I guess too much down time between steps? The back has a potato chip effect. It is flat or it is bowed inward. Which would be INTO the body of the guitar. I need it to bow out as all guitars do. Is there a fix? Heat and weight and time on my radius dish? 15’ radius is the goal but at this point I am willing to settle.

Any guidance?

[charles Tauber]

What was the relative humidity at which the back was braced, what was it when you glued the back on and what is it now?

[Skarsaune]

What Charles said.

[ClaptonWannabe2]

Originally Posted by charles Tauber

What was the relative humidity at which the back was braced, what was it when you glued the back on and what is it now?

SE texas so high for ali.

Back is not glued on yet. I'd like to fix before the box is closed

[charles Tauber]

You are learning first-hand, as many of us have, that up to, and including, assembly, one must maintain a constant relative humidity. Wood moves and changes shape in response to changes in humidity. Trying to build and assemble in varying humidity levels is like trying to hit a moving target: components change shape from one day to the next. Accurate joinery is impossible when the components are constantly changing size and shape.

It doesn't have to be complicated or expensive to maintain the humidity levels and can involve something as simple as taking components out of a maintained closet and putting them back when one is done for the day.

[cobalt60]

Can you post pics? I'm having trouble understanding... if your braces are tall enough, the back simply should not be able to reverse its curvature without catastrophic failure.

[JonWint]

control humidity, remove braces, control humidity, reinstall braces, control humidity, attach back to rim, control humidity, finish guitar

[charles Tauber]

To return the back to its original shape, just return the humidity to what it was when it was braced. You don’t need to remove the braces and re-glue.

What you have created is a mechanical hygrometer: it arches one way for lower humidity and the other for higher humidity. All it needs is calibration.

[ClaptonWannabe2]

Think I’m gonna weight it and put it on flat tiles. Let it get hot. Muggy rain I think is what got it while in garage. If that works, do same with 30’ radius dish. That work? Put it in 15’ dish and if I get anything less than 30’ radius I’ll be happy.

I’ve thought of misting the wood with water before this. Avoiding the glued braces and applying heat like a clothes iron.

[charles Tauber]

Trust me on this: been there, done that.

You don't need to do any of that. You need to reduce the humidity level of the guitar back to the level at which it was braced. If you do that, it will return to the geometry you had when the wood was at that humidity level.

The first rule of woodworking is that you can not stop "seasonal" movement of wood: one's design must accommodate it. The way guitars are designed and built violates that rule in a variety of ways, one of which is gluing long-grain braces across the grain of another piece. The two "fight" each other, resulting in the change of shape you are experiencing when one moves - expanding across its width - while the other doesn't - along its length.

Use this experience to understand first-hand how wood moves in response to changes in humidity. Understanding that is essential to any fine woodworking. Wood is a highly anisotropic material, meaning that it has different properties in different orientations. One of those properties is that when it is subject to a change in moisture content it expands and contracts by different amounts in each of the three primary directions - length, width and thickness.

[John Arnold]

You need to reduce the humidity level of the guitar back to the level at which it was braced. If you do that, it will return to the geometry you had when the wood was at that humidity level.

A sunken back has dried since it was braced. The back has shrunk, but the braces are restricting it on the inside, hence the sinking. Your choices are to increase the moisture in the back, or to rebrace.
The reason this happens is because wood moves very little in the grain direction, so the braces are not changing length. At the same time, the back is trying to change its width.
With that in mind, you do not have to worry about the moisture content in the braces, because it will have relatively little effect on the arching.

[Alan Carruth]

What Charles and John said. The braces don't change length much with changes in humidity, but the back itself gets wider and narrower, depending, so the back can become either concave or convex depending on whether the humidity went up or down. The only real solution is to get the humidity under control. Nothing else works.

[charles Tauber]

Originally Posted by charles Tauber

Trust me on this: been there, done that.

Maybe, don't trust me on this. ;-)

Originally Posted by John Arnold

... increase the moisture in the back...

Thanks, John.

[ClaptonWannabe2]

Work in progress. I’ve moved some equipment inside. All loud or dirty work is done outside. Sanding routing planer etc. then move pieces inside. My error was glue up. Leaving the glue to dry and clamps outside due to room restrictions is just not acceptable here is texas.

I believe I am going to have a very flat guitar. 30 foot top AND back radius would be ok at this point.

[Neil K Walk]

I’d listen to Charles.