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Well, this is certainly not a scientific analysis, but I think I've come to some conclusions about HHG and its "tenacity" .
I ust spent an hour "deconstructing" the braces that I'd previously glued on a Koa back wiht hot hide glue - the short story is that due to humidity swings in my shop (maybe a more accurate statement is due to my gluing a back when humidity was too low... )the back started to get a reverse arch - so... thanks, Hesh and others for your guidance - time to start again -

I'd have simply gone after it wth heat but I was a bit worried about the back joint, so I used the "hair dryer" only sparingly. When I did, the braces came off pretty easily - but near the center joint....... forget it.

This stuff is indestructible. And I think I overbuilt the back braces a bit too, so.... chisels, etc. will be in use tomrorow to get the back cleaned up. But I'm convinvced. HHG is plenty strong for me. (And I really like the smell - reminds me of Breugel, my old Bouvier, just after he got soaking wet, and came in to sit in front of the fire)

Corky,
I am a bit confused. Your said you glued the braces on the back when the humidity was "too low" and your back got a reverse arch. I would have predicted the opposite. I would think that if you glued braces on your back at a low humidity and then the RH came up the back would take a arch in the direction you want. If fact I thought that Martin did just this in the old days to get a curved back or top. The braces would restrict movement on the brace side and the wood would expand on the side away from the braces arching the top or back. Maybe I don't know what a reverse arch is. What do you mean ?
Link

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Cut to size.....Beat to fit.....Paint to match.

I agree with Link. From what I've experienced so far with tops, if they are braced in normal RH and then hit by very low humidity for example 30%, they will take a serious reverse arch in a matter of hours. The only quick fix is to put them in the bathroom and shock it in the opposite direction (for example 65%), otherwise at 45% it will take a long while to fix.
No idea about backs, I attach the braces to the rim first.

Reverse arch = it does not hold water on the braced side.

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Build log

The following is my opinion
HHG is very strong, but it's also hard. Other glues are also very strong but have some dis advantages. The way my twisted mind understands it is it's like having tiny sponges inside the glue joint.

Anyway, heat alone isn't the way to remove HHG since HHG has pretty good resistance to heat. You need steam. With a little heat, as well as steam you can take a super strong HHG joint apart almost effortlessly.

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http://www.brackettinstruments.com/

HHG is not that difficult to take off, but it does take a bit of heat. water does help . it takes a bit longer to loosen than tite bond.

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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 too must agree with Link. Seems to me there was something else going on there to cause the reverse arch. Did you pre-radius the bottoms (glue surfaces) of your bracing?

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Bill Hodge


One does not simply, own enough guitars!

Thanks all, for your responses.

So...I did indeed have a reverse arch - e.g. the back didn't "hold water" on the braced side.

I did radius the braces prior to gluing - so the arch from side to side was the normal desired arch. Where the reverse arch came in was the top to bottom axis - after a couple of weeks the center strip seemed to exert force on the back to cause it to arch in the opposite direction, creating a "pringle" shape to the back Humidity? Poor technique? (Definite possibility - I deviated from the usual of gluing one continuous strip of spruce on the seam - opting for using several pieces, placed in between the brace positions.) I don't think I'll do that again.

Either way, sthg was indeed awry.

I glued it in the middle of winter, when the humidity was quite low, given my inadequate humidifying equipment. The reverse arch became pronounced in spring, once my central heat was off - and humidity rose to about 50%

I hear you on the steam - don't know why I didn't think of that - I know that's how I've removed necks a couple of times. Will do in future.

So, anyway, I've got all the braces off - ready to start again.

Interesting then... seems to be the other way around it usually happens, at least with spruce.

There really isn't anything wrong with using separate pieces over the seam.

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Build log

Also, the cure time will affect the dismantling immensely. If you hit it with heat within 24 hours, it will be way easier.

I had to take off a fingerboard that I had glued on with HG. (Neck set issues). Mine had been cured for a long time and it took a LOT of heat and steam to take that sucker off. I was shooting water in the crack with a syringe, heating with a clothes iron and prying with a thin palette... Any questions or doubts I had about HG were quickly dispelled at that moment. I forget how long it took me just to take that fingerboard off, but it was no walk in the park.

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Corky,
Your discription of the shape as being like a pringle is great. I get the picture now.
That seems odd. I can't imagine your center strip having enough power to cause that. Is the center strip spruce ?
I would think that spruce is so soft it would not exert much power and have a lot of give when it pushed into your braces. Plus bending something against the other bend is hard to do. Did this length wise bend happen after the side to side arch you put in ? If you take a piece of sheet steel or paper or any thin sheet material and put a arch in it one way and then try to bend it in another arch 90° to the first one to create the "pringle" shape it is very hard to do. The first bend greatly stiffens the structure against a bend in the other direction. It usually takes a lot of heat and pressure to get wood to do that or lots of thin laminations. I think there must have been more going on than just the low humidity. I am not doubting you I am just puzzled although with wood it can have a will of it's own and defy the "rules" at any time. Well at least you have more stable RH now and can proceed. Please follow up with your experience and how it comes out this time and what you observed.
Link

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Cut to size.....Beat to fit.....Paint to match.