I finished building a 000, with an EIR back, last winter. Two cracks appeared on the treble side of the back (I was spraying it when the temperature was too way low). One crack is 11" long and 2" out from the center line. It runs from near the top down to about the lower back brace. The other is 3" out from the center line and runs 3" up from the butt. Both cracks are tight with no visible separation, although I can see a small amount of lacquer on the inside where it wicked through the crack. Before I noticed the cracks I did attempt to repair the finish with amalgamator, but there is still quite a visible crack line in the finish.

I thought I would run my planned fix by some of you with more experience with this type of repair than I.

1) Strip the back completely. Try to get some lacquer thinner to wick into & through the cracks to clean them out. I thought that stripping would be the best, & ultimately the easiest solution, because this is a relatively new finish, it would be easier to, & prob look better, than trying to repair the existing finish, the fact that the cracks run the full length of the back, & the ease of access from the outside on a 4"deep guitar. I would then let the guitar dry for a week or two - to loosen the cracks up a bit.

2) Fill the cracks from the outside with THIN Stew Mac "super glue". I am concerned with the listed 3 to 5 sec cure time for the thin vs. the more workable 10 to 25sec cure time of the medium variety.

3) Glue some EIR re-enforcing cleats to the back between the braces with magnet clamps.

4) Refinish the back.

I originally posted this to the UMGF list back in May, but got very few responses. Frank Ford posted a similar use of Super Glue for a smaller back crack on a '66 D-28. He says he used "thin viscosity cyanoacrylate" (super glue), but this was a much shorter crack than I am dealing with. I was hoping Frank Ford would weigh in with his opinion & suggestions, but he didn't. Maybe I can get him, & others, with more experience to weigh in with their thoughts, suggestions &/or alternatives.

Bob Krueger

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Can you add pics?
I've tried that. I am a photograher by profession. Pictures don't show anything - no contrast.
What if you put a cold light inside? I've been a photographer too.
Here are 2 previous photos that, as I said really show very little, if anything. One is of the outside of the back. The other through the soundhole. I feel the verbal description is much more informative. I don't want to get hung up on a photo thread that doesn't really address the problem.

Hi Bob

I don't know If I am more experienced (just 10 years repairs of mostly guitar-like instruments, only a few guitars). But anyway, here goes my opinion:

I think your steps 1-4 are just fine. Without knowing if you thought of that: grain orientation of the cleats should be a few degrees off the back's grain orientation.

I don't know how much this is already state of the art in the US: for handling things inside a guitar body, sometimes I place a webcam and a cold light inside the guitarbody to see on the monitor what I am doing inside. Correct placement of the light source is very important for a decent image (ok you're also a photographer, so you know that already). The light: 5 Watt should do it, 9 Watt are plenty. Even if it's called "cold light", better don't let it get in direct touch with the guitar. Depending on the webcam, you get along very well even with indirect sunlight from outside. With my webcam a direct, relatively strong bulb-light from outside (directed to the top of course) gives me also an image, but not a good one.

BTW: To me, the two pictures speak a clear language, they just tell me what you write in the opening post. They address the problem very well. Probably a third photo from outside, with a cold light inside would show - just nothing! This also would be a piece of information, although not absolutely (or not at all) necessary ;-)
I think it's always good to have a picture and a verbal description.
Hi Bob!

After cleaning up the crack, I'd use FRESH HOT HideGlue. A wider latitude of working time than super glue, and much easier clean up with a warm damp rag. Then clamp to close and secure the crack. (dry run this first!). While clamped, apply the cleats, with the cleat grain crossing the guitar.
Use magnets to "clamp" the cleat in place. Best to keep warm the area of the crack...before applying the glue to prevent premature setting up of the glue.
Perfect advice, but I'd use slow drying CA, maybe gel. It's less prone to melt by temperature. Just my opinion.
Sorry to be slow responding here.

Thanks for posting the pix. It does look like the crack is somewhat wider than I what I would call "tight." Seems like I see a bit of an opening from the outside, but more important is your statement and photographic evidence that lacquer penetrated the crack and is visible inside.

That's kind of a big deal. The crack is definitely contaminated enough that I simply would not trust hide glue to adhere well. In fact, I think that CA is the only stuff that would be likely to stick because of its chemical relationship to lacquer.

That said, I also like the medium viscosity for gap filling, which is partially the issue here. So, I'd stress the crack open by pressing from the inside, and run in as much thin CA as I could, and follow it with medium wherever it will go. That way, you get the benefit of the capillary attraction of the thin to go way up into ends of cracks, and the gap filling properties you need from the medium.

I'd strip and refinish after the crack repair, and I would not expect such a crack repair to be invisible even if the glue holds well. Later, it's likely to "translate" through the new lacquer as the finish ages. Sad fact of life, but that's what we tell customers to be aware of. As to cleats, their design, size and material are subject to personal preference, I think.

Thanks for your reply & suggestions. I think what you are seeing with the crack that might look like seperation is the really the crack & seperation in the lacquer finish & not a seperation of the back wood itself. After you posted your response I took the guitar into my darkroom ( mostly unused in the digital age) and tried to shine various mag flashlights, & finally a 60 watt bulb through the crack. None of these lights would actually show ANY light coming through. This, & scanning it with various 8x to 20x magnifiers is what leads me to say that I think the crack is tight. I did not actually notice the crack at all until I noticed the crack in the finish that would not fill in as I sprayed succeeding coats. It was still there, or reappeared, after I finally buffed it out. It was then that I attempted to fill the crack with a lacquer/amalgamater mix. It was only later that I noticed that that brushed on mixture had wicked through the crack to the inside. These factors are what leeds me to believe that the crack is fully tight with no openings or gap.

I agree that the crack is contaminated. I thought that if I stripped (BIX - now Jasco) the back first I could at least partially clean the crack with lacquer thinner, acetone, & alchohol before I glued it. I am worried that the amalgamater/lacquer mix might have fully contaminated the crack. If I don't fully strip the back first I would'nt I need to at least need to open the an area on either side of crack so that the CA would wick through?

I haven't got a lot of experience using CA, esp. on wood. So I will have to defer to your experience as to whether to use thin or medium viscosity, as well as whether to stip the back before or after gluing. It sounds like it would be a lot easier to use medium due to its working time, but as I said, I am concerned about its ability to wick through the crack. If I try medium & it doesn't wick through then I'm stuck - or maybe not. Should I get both viscosities, & try the medium on a short section to see if it works/wicks. If it doesn't, can I then go over that same section with thin, or will it be sealed?

I don't expect the repair to be invisable. When you say that 'it's likely to "translate" through the new lacquer'. Do you mean just visually, or will the lacquer physically crack along the original crack/glue line?

Thanks in advance for your help.

Bob Krueger
"Do you mean just visually, or will the lacquer physically crack along the original crack/glue line?"

Either, both, or (rarely) not at all. It may sink or rise on the glue line, or crack over that line with time. Or maybe not.

The whole deal with CA is that it cures by polymerization rather than solvent evaporation. The thin stuff will run in where the thick won't be able to, so you MUST use the thin first to be sure it goes all the way where it can do some good. Follow with medium to fill gaps best, if it will flow in.

IF you do chemical stripping, or other solvent stuff gets into the crack, the likelihood is greater contamination because you can't effectively wash it all out with solvent.

Thanks again. Should I do something ( sanding through finish - etc) to somehow gain acess to the wood portion of the crack? Or should I count on the CA passing through what opening there may, or may not be, in the finish?

Bob Krueger
If the lacquer has wicked through the crack and you can see it inside it was cracked before you applied the finish.

I would use the thin super glue and wick it the full length or the crack and try to build it up with the thin stuff. Then use the thicker to finish building up the crack let it cure and scrape the glue very carefully off and sand flat and buff to finish.



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