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As a result of a lapse in thinking, I cracked a EIR back slightly. The crack runs with the grain for about 4 inches and at one small point (maybe 1/4" long), you can see light through the back. I think it is salvagable. Do I fill the crack with CA or mix some EIR dust with epoxy? What can I use to thin the CA or epoxy to insure it flows all the way through the crack?

ca will work well. the water thin stuff will wick into the crack with no penetration problems. the same can be said for hot hide glue, but you may have to thin it depending on how strong you brew it, and make sure you have warmed the wood before you apply it. you will be able to see the glue coming out the opposite side of the crack.

Sounds identical to a crack I had happen in a cocobolo back. It wouldn't quite close all the way. I used medium CA to fill it. Afterwards, I could hold the back up to the light and see a glimmer through the crack. But it was invisible otherwise. This particular guitar was a prototype, so I wasn't too worried about appearance. So, I just braced up the back and installed it. That was about a year and a half ago. So far so good. The crack is still invisible. I probably couldn't even find it unless I stuck a light inside the guitar and moved it around until it was directly underneath it.

I think an epoxy repair would work equally as well, if not better. You can probably push it into the crack with a spatula or something similar.

Best,

Michael
Michael McBroom38730.450162037

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   If you sand the back and let the dust fill the void , then CA it you can do a great repair
john