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I had a guitar I built come back with a couple of top cracks that the owner said occurred when the bridge came loose, although looking at the abused case, I have to wonder what caused the bridge to come loose. One long crack runs toward the butt of the guitar from the 6th string bridge pin hole for about 7 inches (see pictures). The other crack is in the center seam and runs about 2 inches across the bridge area (can't really see this one, but it is there!).  Since the owner (a relative) was going to leave it with me for several months, I opted to remove the bridge, remove the finish from the top (kind of regret this step now), humidify the guitar to get the cracks to close up, fix the cracks, and then refinish the top.  I used thinned yellow glue, pressed on the inside of the guitar near the cracks, as I attempted to work the glue into the cracks with my fingers (at least I thought I worked the glue into the cracks). Actually, I think it was yellow glue and not hide glue. Unfortunately, I didn't write it down and it has been too long for me to remember!  I then clamped it up and let it dry. The next day I made some small cleats that I glued (with hide glue) across the cracks in a couple of locations on the inside.  I then hung it up in my humidified shop where it stayed for a few months until I could get back to it to reapply the finish.  When I came back to it last week, the cracks had opened up again!  I am guessing that I didn't get glue into the cracks like I thought.  I humidified the guitar and am able to get the crack to close up. I removed the cleats on the inside and am now wondering what to do to really fix the cracks!  I would like to use hide glue, but I fear that I won't have enough working time to get the glue into the cracks before it sets up. I probably worried about that last time and is why I think I may have used thinned yellow glue the first go around.  Any suggestions on how to make sure the cracks are really fixed this time?

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I would use un-thinned yellow glue, because now that the glue joints are not clean anymore (traces of the last glue), hide glue could be not that efficient. Un-thinned yellow glue will have strength, and you'll have plenty of time to work it in, using a piece of paper, or an air blower. Then apply more apply more cleats than last time.

You're going to have to keep the guitar humidified to keep the cracks closed.  If you hang it up on the wall it is going to dry out and the cracks open.  Walls are not good places for guitars.  If it were my guitar, I'd get it properly humidified and cracks closed up, spray a coat of finish on it and then work hide glue in from the outside of the guitar.

Good points Glen. After I closed up the crack with proper humidification, even with the guitar just sitting on the bench in my humidified shop for a couple of days while I was thinking about what to do, the cracks started to open up again.  Is the "coat of finish" you would put on just a sealer coat or a couple of passes of nitro, not intended to have much thickness?  With the short working time of hide glue, how do I ensure it will get into the crack thoroughly?
I recommend the finish just as a sealer coat so that when you work the glue into what is left of the joint, it doesn't make a mess on your bare wood top.  If it is opening up just sitting there, I'm wondering if there isn't more going on.  If the cracks close with "humidification" and open in a "humidified" shop, it makes me wonder what the humidity level in the shop is compared to the method you are using to humidify the guitar.  The guitar is going to have to reside in the humidity level where the cracks closed up (at a minimum), even after re-gluing and re-finishing.  If that is in a plastic bag with wet sponges in it, it may not be practical.  When a guitar built in a controlled environment (say 45% rh) is moved to a dry place, it cracks.  To fix it, it is brought back up to proper humidity and whatever crack is still showing is filled with glue to keep crap out and form a bond if possible. The glue won't hold it together...much.
Yes, to close the crack I bagged with wet sponges for 3 days. So the humidity level was definitely higher during humidification than my normal shop humidity. My shop is at 40% - 50% humidity. The guitar was built in a 40% to 50% humidity environment.  It sounds like, and it makes sense to me, you are saying to let is sit in a 40% to 50% environment (not on the wall) to close it as much as it will go and then fill it with glue. Am I understanding correctly?

I guess my thinking was that when the first crack occured is was still tight, so by humidifying it until it closed tight again (even if it took only a few days to get it there), then I could then glue it shut, cleat it, and expect it to stay, as long as the guitar is humidified to at least 40% humidity after it is repaired, like it was when it was built.  This may be flawed thinking here, so feel free, anyone, to set me straight.  Thanks!

As long as you've stripped the top, why don't you take the opposite tack and dry the guitar to open the cracks and then add some spruce slivers in the cracks? Open the cracks a little more by cutting them into a V shape, then fashion a corresponding sliver of spruce to fill the gap. Glue it up, sand it level and shoot the top.

 

Not that you need my backing but I was thinking that I would probably use that approach too. If the crack won't stay closed, than it seems that the top wood is stressed and , perhaps, wasn't as dry as it might have been when the guitar was built. Now the wood wants stay in the "place". Splinting it should insure that it doesn't crack again.

 

 

Mark, I have not taken this approach before. By a " V-shape", do you mean to widen the crack at the top surface relative to the inside surface to make a V and fill it with a V-shaped sliver?  What is the best way to "fashion" a sliver of spruce?

It was either Sloane or Ibex who made (or still makes) a little brass jig with sandpaper to shape a crack splint.

Picture two small plates hinged together at the bottom, forming a variable "V'.  The sandpaper is on the inside of the 'V' and the angle can be locked.  A small piece of spruce, the length of the crack+, is drawn-through the 'V' and eventually sized-down to whatever matches the vee-cut you made in the top with a razor knife.  

The angle of the 'V' should be as steep as you can get-away with. The splint is glued-in and cut down to surface when dried. Works pretty well but takes some practice. 

Yes, you have the idea. I don't know the best way except to say it may take several attempts to get the right sliver to fit.

Our winters are very dry so January and February are very busy months for me repairing cracked tops and loose bridges ...I usually cut splines by planing one edge on a slight angle and then cutting the other side with a sharp knife using multiple passes.

To get glue into a crack I put a bead of glue over the crack and then clear packing tape over top sealing around the glue, then I rub the glue into the crack, it can only go into the crack. There are some old finishes that will lift with the packing tape so use your own discretion. 

 

I make cleats using a small plug cutter and then thin them out. I place them by putting a rare earth magnet on the outside and securing it with masking tape. I then use paper glue the (kids stuff that comes in a tube like chapstick) to stick the back of the cleat to the second magnet and put fish glue or wood glue on the top. Reach inside the guitar or use a tool to get it opposite the outside magnet and it will jump up into place. 

 

Regarding humidity I keep mine on the low side in the winter assuming that the guitars that come in are going back to less than ideal conditions when they leave and a sudden big change in humidity is worse than a small one. 

If this guitar was on my bench I would spline the crack. I've used shaving from a hand plane as spline material too.

 

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