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Cracked top repair
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=46760		 Page 1 of 1
Author:  Imbler [ Tue Dec 01, 2015 1:08 pm ]
Post subject:  Cracked top repair
Hi, just glued the back on my guitar and when removing the top (I glue top last) it cracked a two or three inches from the waist going down into the lower bout. I think it will be un noticeable if I fix it correctly, and it is an expensive german spruce top, so I want to salvage it. My plan is to water down some fish glue, work it into the crack, let dry, then install some small cleats on the back side. I don't foresee even clamping it as it fits well with the crack closed. Does this sound right? About 50 percent dilution on the fish glue sound ok?
Thanks for the help,
Mike
Author:  Barry Daniels [ Tue Dec 01, 2015 8:48 pm ]
Post subject:  Re: Cracked top repair
No, no. Use full strength glue. Dilution will just weaken it.
Author:  DannyV [ Tue Dec 01, 2015 9:01 pm ]
Post subject:  Re: Cracked top repair
If you just glued it on and it's already cracking, you could be in for a crappy ride. Do you have dehumidification? It is very important that wood is stored in 40 - 50 % humidity for some period prior to it becoming a guitar. Even more so if you live in an area that is prone to extreme lows. Just sayin'.
Author:  Imbler [ Tue Dec 01, 2015 9:59 pm ]
Post subject:  Re: Cracked top repair
Danny, no I work in very controlled humidity. I glue on the back, wait 24 hours, then remove the top and clean out any excess glue from the back glue up. When I was removing the top, unbeknownst to me, a single errant drop of glue had tacked one part of the top in place. As I was pulling the top off, I just put too much force on it and cracked it starting at the point it was tacked in place. Just bad luck (I've never had any glue run down or drop on the top like this single drop fell, if it has gone down the side it wouldn't have made it a quarter inch.) Plus, I obviously put more force on it than I should have, but where I was pulling the top off really leveraged itself across the width of the guitar to the other side where the split occurred.

Mike


DannyV wrote:
If you just glued it on and it's already cracking, you could be in for a crappy ride. Do you have dehumidification? It is very important that wood is stored in 40 - 50 % humidity for some period prior to it becoming a guitar. Even more so if you live in an area that is prone to extreme lows. Just sayin'.
Author:  Imbler [ Tue Dec 01, 2015 10:03 pm ]
Post subject:  Re: Cracked top repair
Barry Daniels wrote:
No, no. Use full strength glue. Dilution will just weaken it.


Thanks Barry, while waiting for replies I cracked a few scrap test pieces to practice. I tried some diluted glue (about 50 percent), and it was practically worthless. I then tried full strength, and was able to work it into a test crack.

Like they say, practice on scrap! I have lots of cutoffs from tops, and now with a little practice, I feel a lot better about attacking the real top.

Your advice is spot on,
MIke
Author:  DennisK [ Tue Dec 01, 2015 11:24 pm ]
Post subject:  Re: Cracked top repair
Yep, just glue it and continue. I've done it a few times... oops_sign

I use hot hide glue, with a little more water than my standard mix, but still within full strength range. With everything nice and warm, apply glue liberally to both sides of the plate, completely covering the crack, and then flex the crack open gently to vacuum the glue into it. Squish at it with your fingers and flex back and forth for a while to get it worked in there as good as possible. Hold it closed by hand for a minute or two, and it should be good to go.

Stretch some tape to clamp it if it makes you feel better... but be careful not to accidentally peel the crack back open and screw up the bond in the process.
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