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Ii've got a PITA coco back that is giving me fits. i've found and repaired about 4 cracks. Initially i used super glue and that worked out fine on the first series of cracks. However, for the largest crack I decided to use hide glue (Behlen 454g) and i cannot get the crack to stay closed. even applying a small amount of stress re-opens the crack. I plan to cleat the cracked areas as soon as I can get this last crack properly repaired. Today will be the 3rd time attempting to close it and I'm not sure what I'm doing wrong. Here's what I'm doing:

-Heating the area with a hair dryer
-brushing in HHG (145-150 degrees)
-lightly clamp and let sit overnight

Each time the joint has failed. Should I be using hot water to reactivate the old glue inside the joint? I'm sure this is more a learning curve with HHG more than anything else as the other repairs went well. This guitar and all its warts is definitely not going to be a seller but regardless I would like to get this done right.


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Cocobolo can be quite oily. May have to clean the crack out before gluing.

Joey, I may well be mistaken, but using heat could be your problem. You’re artificially drying out your cocobolo, and then when it re-accimatizes, there are probably some weird stresses happening.

Is there a serious gap, cause hide glue isn’t good with gaps.

If there’s a “large” crack, you may need to add a sliver/wedge of cocobolo and then glue it. Forcing a bad joint with clamps can only do so much.

A couple of thoughts. Wish I could help more.

Steve

I would clean out the hide glue with water and dish detergent and then rinse with clean water and let it dry. The detergent will remove some of the oil from the surface (eventually it will migrate back). Then I would apply the super glue and sand over top of the crack (creating some dust) while it is still semi wet. Assuming the crack isn't so wide it needs a splint, the dust can fill small voids and when you add cleats to the back side it should hold up O.K.

hhg and cocobolo never worked for me too oily I won't use it anymore

_________________
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 cleaned it out, let it dry and CA glue worked to close it up. HHG seems to work fine on the back reinforcement strip and bracing, but not to itself for the crack.

I had the same problem with coco back in jan used white PU (poyurethane) gorilla from h, depot clean with q tip and acetone fill with glue and use light vln making aluminum clamps. or HF plastic bar clamps or ???

The surface of the cocobolo was likely degreased when the guitar was made (Acetone or naphtha wipe before glueing the back bracing and finishing the outside) so the hide glue and cleat will stick, but maybe not inside the crack.

Inside the crack still has those wonderful oils that make cocobolo so challenging. If the finish will tolerate it, I'd clean the cracks with acetone, alcohol or naphtha first, and then try glueing. Make sure you know what the finish can tolerate first.

Splicing was also a great recommendation if there is too much tension in the back—it may have been braced at too high of a humidity and really wants to shrink a little. No sense clamping hard and putting that tension back into it, as it'll remind you of its intensions sometime down the line.

I just used one heavy duty bar clamp very lightly clamped with a couple spring clamps and a couple wood strips lined with wax paper to hold the crack together.

I wish I would have seen this post before I glued the back braces on. Oh well, at least HHG is easy to reverse. I just went to carve the back braces and just the light pressure from my block plane undid the braces from the coco. Oddly the other back I did with HHG/mahogany/coco carved fine before getting to this back. I decided to plane the other braces to see what would happen. 3 of 4 failed so I removed the 4th, cleaned them all up and then re-glued with Gorilla Glue.

Another day, another lesson learned the hard way. I figure I may as well go back and take the braces off the other back and and re-do it as well, much easier to do it now rather than later.