Double top glues ?

[Chuck Morrison]

I've built quite a few double top guitars (sheets of spruce/cedar with Nomex) using T-88 epoxy with a lot of success. These tops have been measurably lighter and stronger than their solid wood counterparts. They also take considerably longer to build. I've used techniques as described by Alan Dunwel on Robbie O'brien's videos. I can't complain with the results, but I really don't like epoxy, especially the cleanup, and there is a huge amount of waste involved in the process. The vast majority of the glue goes in the trash. So this last time I used gorilla glue instead. My test patches looked OK but on the actual top I had several areas where there was too much foaming. I did not apply water to anything but it still foamed into the honeycomb. On the assumption that I just had too much glue in areas I made sure the other side didn't get any extra. Of course one issue with the second side is that you can't see into it to see how well you did. If this doesn't make sense watch the videos.

OK, long story short, I also did a small layout using fish glue. It seemed fine, but I don't know if it is. I've heard that one should never use water soluble glues for double tops. I'm not sure why, since no one has ever elaborated. I'm sure there must be a reason, anyone know ? I'm guessing either long term viability or wood swelling due to the water in the glue.

On the next one I guess I'll try Bolder Bond. It's supposed to be low foaming and about 20% stronger than GG. Anyone tried it ? Titebond also has a PU glue, how does it compare to GG ? Especially the foaming part.

[Clay Schaeffer]

Why is foaming a bad thing? It shouldn't make any difference in the weight and might even add to the stiffness.

[Alan Carruth]

My guess is that water soluble glues won't stick as well to the Kevlar/phenolic Nomex. Other than that I can see no reason not to use it in a guitar. The Kevlar stuff is water and heat resistant, which is useful in automotive and aviation applications, but we don't need it.

[Chuck Morrison]

Why is foaming a bad thing? It shouldn't make any difference in the weight and might even add to the stiffness.

I can't imagine that an area filled in with PU foam would have the same acoustical properties as one filled with just air. but if they were all filled in, what would that do ? I don't know. i wonder if PU' foam's excellent sound deadening properties may come into play. That's what I worry about with some areas foamed in and some not, relative dead spots built into the top. I don't know that for a fact, just speculating.

[Chuck Morrison]

My guess is that water soluble glues won't stick as well to the Kevlar/phenolic Nomex. Other than that I can see no reason not to use it in a guitar. The Kevlar stuff is water and heat resistant, which is useful in automotive and aviation applications, but we don't need it.

That could be an issue, although my little test piece seemed to be well connected. However, I can see having some concern for an organic sourced (hide/fish...) glue failing from natural decay over time or coming apart in a hot car trunk. Even titebond could slip with heat, so maybe those are valid concerns too.