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I am early in the learning curve, trying to learn how to use hot high glue for my builds. I glued the back, transverse braces on with hot glue. The back warped on me like a potato chip, especially on the thin outside edges of the transverse braces. Has anybody else had this happen?




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Formerly know as Mandodiddle.

I have a couple of questions:

1) (and the most obvious) I am going to assume you glued the braces on under humidity control? I (and I think most others too) work in a controlled 45% relative humidity environment). This is absolutely crucial.

2) You seem to shape the brace before glueing? I prefer to shape the brace after glueing the brace. Also, that is some pretty radical scalloping, IMO, which may contribute to the problem. (disclaimer: I don't build steel string guitars, so there are probably others who are better suited at advising on that. But personally, I would probably go for a more even taper at the brace end.)

3) I am going to assume you glued a curved brace in a matching radius dish?


What I would do to fix it: take off the braces (planing them down to almost nothing, remove the remainder with wet cloth and clothes iron. Let the back settle back down. I would probably keep it between 2 flat plates for a while. Possibly weighted down. Let the back climatise at 45%RH for a good while, and rebrace. And use a gentler taper at the brace end.

Good luck!

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http://www.vanguyseguitars.com

I wouldn't worry about the warping. It looks like you're working in uncontrolled humidity and glued them while it was low, and now it has gone up. That's ok. Gluing braces in high humidity is what will kill it. Just mash it down when closing the box. Ideally use spool clamps and put the ones at the widest points first, then the ones at the center line, then half way between those, and so on. That way the pressure is spread out, same as it would be if it had been glued on in low humidity and brought into high humidity afterward. In extreme cases the expansion pressure can pop glue joints, but I don't think it will here. The warping just looks like a lot because those brace ends are so thin and flexible. You've made a wood strip hygrometer

Like Maarten, I would recommend keeping the brace ends taller in the future. I usually have them 1/8" to 3/16" where they notch into the linings, whereas yours look like 1/16" or less, and are carved down to that height a long way from the edge so they'll be very flexible. Back resonant frequency will be much lower than the top, whereas it's better to have it a few semitones higher.

If you want to rebrace now, and don't have humidity control, then don't remove the braces entirely. Instead carve the existing braces down to 1/16" and glue new braces on top. That way the existing braces will hold the back fibers in the "glued in low humidity" condition so it won't crack in the future. It will look like a fruit roll-up while when the braces are carved down thin, but again that's ok. Just squash it back to the proper radius with the new braces.

While that is true for sure, I would add that the only time I have actually had damage due to humidity problems was not when it was too dry (I'm careful that way), but when it was impossible to keep the humidity low enough (long period of heavy rain, during winter, and the heating went out). So the guitar was too humid for too long, and it pretty much exploded along the peones keeping the top on the rim (there was no binding yet at this stage, so all the end grain was still exposed).

So I'm not sure I would want to continue with braces that I knew were not glued on at the right humidity. Regardless of whether it will cause problems later on, it will certainly introduce stresses in the guitar, that I would not want to build into the guitar. But I agree that from a structural point of view, it will (probably?) be OK.

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http://www.vanguyseguitars.com

If you can fit the back to the sides with light pressure I would not worry about the "potato chipping". The thinner ends of the back braces will allow the back to flex more easily with changes in humidity. It looks like a fairly small instrument, which again is a plus in this situation.

most guitars are made with a back radius designed into them for strength and this helps allow movement in humidity changes . It has nothing to do with hot hide glue . Wood will expand and contract with humidity. Brace will fight this movement and the back with bow and sink as a result.

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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

Bill this is the ole causation vs. correlation problem with determining what actually went wrong.

It's not the HHG or the millions of instruments built with HHG that did not have their backs move at all when the braces were glued on with HHG. No need to be looking for LMI glue either as I saw in your other post.

Maarten's post above is excellent and covered what you really need to know here. I strongly suspect that the humidity control is lacking in your shop and/or process.

Did you heat the top in an effort to extend open time? Could have dried in out a lot.

From this stage, completion of the instrument still involves several man hours. It is well worthwhile to spend a couple of hours removing and replacing those braces, no matter who is right on the question of, "Will the curled back cause problems?" Why take the chance over a couple of hours of extra work? I would remove them, make sure the back has time to settle at the proper humidity level, and carve the ends in a more traditional fashion. I agree that the ends are too thin and scalloped too far in, but not the cause of the curling in the center of the back. I also agree that hot hide glue had nothing to do with this problem.

Nice to see quality consciousness and folks willing to do it over until it's right. Thanks JTB for a much appreciated post and timely too.