Crack repair

If you have a string instrument of any kind that needs fixing, a mistake you made in building a new instrument that you need to "disappear," or a question about the ethics of altering an older instrument, ask here. Please note that it will be much easier for us to help you decide on the best repair method if you post some pictures of the problem.
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Dick Hutchings
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Crack repair

Post by Dick Hutchings »

I made this repair with thin CA. As much as I wanted to use something else, this crack was just to tight. My question is, would you go further in an attempt to hide the line.? I obviously did a better job of aligning the wood towards the lower bout. Tint some CA?
Crack.jpg
Repaired
Repaired.jpg
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Barry Daniels
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Re: Crack repair

Post by Barry Daniels »

Glu-Boost with vintage tint kit would be my choice.
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Michael Lewis
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Re: Crack repair

Post by Michael Lewis »

Hot hide glue and rub it in. A bit too late for that now.

A serious point about cracks under finish: hide glue shows the least of any glue. CA can make a structural repair, but is more difficult to camouflage. The issue with the CA is it is nearly impossible to reverse it without seriously damaging surrounding finish, while hide glue can be washed off with water and a bit of heat. I apologize if this is a sore point because you hear it all the time, but it is information that should be in every luthier's arsenal.

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Ryan Mazzocco
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Re: Crack repair

Post by Ryan Mazzocco »

Michael Lewis wrote:
Sat Jan 18, 2020 2:42 am
Hot hide glue and rub it in. A bit too late for that now.

A serious point about cracks under finish: hide glue shows the least of any glue. CA can make a structural repair, but is more difficult to camouflage. The issue with the CA is it is nearly impossible to reverse it without seriously damaging surrounding finish, while hide glue can be washed off with water and a bit of heat. I apologize if this is a sore point because you hear it all the time, but it is information that should be in every luthier's arsenal.
Michael, to be more clear, are you talking about using HHG for cracks in the wood, cracks in the finish or both? I'm all in on using hide glue these days. I use it any time I can. Clearly, it's best for repairing a crack. But I also just recently got set up with GluBoost. I've done a few jobs with it and I have to say I really, really love the stuff. It has made a few jobs that would have taken me a couple of days down to a couple of hours (or less!) I've really only had one problem. I had a guitar in last week that had cracks galore in the top, but they were all only in the finish; nothing structural. One crack went from the treble side of the headblock to the rosette; two cracks from the bridge to the end and one crack that ran the entire length of the top and passed just to the bass side of the sound hole and bridge. (Now regretting giving too much information, but I already spent the time typing it so it would just be a waste to delete it all now.) So, here's what happened. the cracks were really tight but you could feel them if you ran your finger across them. I used the thin fil n' finish and accelerator to fill. Finished great but I believe the fil n' finish discolored the spruce as it darkened the line just a tad perfectly along where the crack was. It wasn't terrible, but could be a problem if the customer is super picky, and I wouldn't blame him if did have a problem with it. Just so happens, this customer was delighted with the results. But, in the future, would using HHG first in a situation like that create a barrier to protect the wood from the CA? Maybe make the glue a bit thinner, apply, and wipe off the excess, then do the fil n' finish?

Steven Smith
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Re: Crack repair

Post by Steven Smith »

I've been using the GluBoost since it came out and I do like it a lot for finish repairs. I prefer HHG for bridge/brace reglues and crack repairs unless it's a very low-cost guitar with a client on a low budget.

One thing I do is that when I fix anything on the finish I tell the owner that I'll make it better, I'll fix the crack, or whatever but the repair probably won't be invisible.

Michael Lewis
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Re: Crack repair

Post by Michael Lewis »

In response to Ryan Mazzocco's question, HHG for structural crack repairs in the wood. Finish is it's own thing, and effects how light reflects. A particularly noticeable problem is discoloration along a crack. This is often due to dirt or some other substance getting into the wood fiber, especially polish, oil, or just a wet rag on the surface to wipe fingerprints away. Also, if a finish crack is left for a long time oxygen in the air can oxidize wood cells and discolor them. Prevention is the best way but we usually see these things well after the damage is done. This is why crack repairs should be done as soon as possible after the cracks appear.

Finish has no grain structure to speak of, so when it cracks it follows stress in it's substrate. This is also true of polyester resin finishes like on Ovation guitars.

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Ryan Mazzocco
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Re: Crack repair

Post by Ryan Mazzocco »

Michael Lewis wrote:
Wed Apr 01, 2020 3:15 pm
In response to Ryan Mazzocco's question, HHG for structural crack repairs in the wood. Finish is it's own thing, and effects how light reflects. A particularly noticeable problem is discoloration along a crack. This is often due to dirt or some other substance getting into the wood fiber, especially polish, oil, or just a wet rag on the surface to wipe fingerprints away. Also, if a finish crack is left for a long time oxygen in the air can oxidize wood cells and discolor them. Prevention is the best way but we usually see these things well after the damage is done. This is why crack repairs should be done as soon as possible after the cracks appear.

Finish has no grain structure to speak of, so when it cracks it follows stress in it's substrate. This is also true of polyester resin finishes like on Ovation guitars.
Interesting you mentioned that. Coincidentally, the guitar I was working on was an Ovation.

Something I'd like to try next time... Maybe put a bit of shellac in the crack first? This could wick in and seal off the wood so that the CA doesn't darken it if it seeps all the way through. the original line may still show where the finish layer actually cracked, and may not deal with dirt, oils and other contaminates to keep them from showing up in the repair, but if I can keep from adding even more discoloration that would be nice.

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Barry Daniels
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Re: Crack repair

Post by Barry Daniels »

If you put shellac in the crack then the CA will not have any place to go.

If you don't want the CA to wick so far into the wood and discolor it, then you might consider using a thicker formulation of CA.
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Ryan Mazzocco
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Re: Crack repair

Post by Ryan Mazzocco »

Barry Daniels wrote:
Thu Apr 02, 2020 9:50 pm
If you put shellac in the crack then the CA will not have any place to go.

If you don't want the CA to wick so far into the wood and discolor it, then you might consider using a thicker formulation of CA.
That's a fair point. I did use the thin CA.

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