archtop crack repair

[Jim Hepler]

Got some cracks in the top of an archtop guitar I made. It received a knock on the tail end. There are a couple of big cracks on the lower bout that close up cleanly and become pretty much invisible with proper alignment. Seems like those will be no big deal to glue. I'm planning to use hide glue unless you folks think otherwise. The crack on the upper bout is also clean and doesn't appear to be missing any wood, but it only closes with quite a bit of pressure on the sides. By pushing pretty hard on the waist I can get it pretty much closed, but I'm concerned that if I clamp it like that it will want to rip apart again. I guess the other option is to put splints in. Eagerly awaiting expert advice. thanks,
jim

[Barry Daniels]

Absolutely use splints after gluing the cracks. Make them small, from spruce, and oriented cross grain. If you keep them small they will not hurt the acoustics at all, but will support the crack a bunch.

Getting a bit of humidity into the guitar may help you close the cracks without having to use excessive clamping pressure.

Only use hot hide glue if you are experienced and comfortable with it. Otherwise, Titebond will give you more time to get things into alignment before it sets.

[David King]

The fact that this top was under some considerable strain might indicate it's time to relieve that strain or at least figure out what's causing it. If it's just a moisture issue that can be easily remedied then fine but if there was considerable shrinkage between when you built the instrument and now I'd worry that this crack might become a recurring problem. Unfortunately it's not like a violin family instrument where you can just pop the top off and re-glue it in slightly from the rim so you may have to add a spline into the crack.

[Jim Hepler]

Barry, When I said splints I meant little fillers strips to fill in gaps on top. I think you are talking about little cross grain patches to use underneath. I'm probably not using the right terminology. I had planned to put some of those cross grain patches on the inside to help hold things together. I did use hide glue on about half of this guitar, which is the extent of my experience with it. I feel confident, but if things seem to go slow on the dry runs, I could switch to titebond. Aside from the open time, do you feel there is an advantage to one or the other for this type of repair?

David, You have hit on exactly what I'm worried about with the one crack. I can squeeze it closed without clamps, but I'm squeezing fairly hard, so the choice is either to fill it with little sticks - I still have off cuts from the top, or squeeze it, put the cross grain patches underneath and hope it holds. For sure we are in central heating season now, and that could be an issue, but the wood was in my house for several years, so it was well seasoned before I built this, and it wasn't hot and humid when I glued it together. I guess I don't know how to really determine the cause of the gap. It could be lack of moisture, or it could be something got knocked out of alignment somehow. I don't think there's any wood missing. Seems sort of odd that the cracks in the lower bout disappear without any force but the other one doesn't.

Thanks both of you for your quick replies.

jim

[Barry Daniels]

Oh, Ok. The splints you talked about are only used if you can't get the crack closed. It is very difficult to get a spline to not stick out like a sore thumb. If you can get the crack closed, it will be much less apparent, or even invisible.

By the way, the cross grain patches inside the guitar are normally called cleats.

If you have to use moderate clamping pressure to get a crack to close, that is probably not so bad. A properly glued and cleated crack should be able to hold up to some pressure, and after the repair, the instrument's stress will equalize and redistribute so that all of the stress will not be concentrated right on the crack. At least that is my theory.

[Jim Hepler]

Barry,
It turns out it does only require moderate pressure across the body and a little bit down (that was the part I was missing) to get the one problem crack to close. Seems like if I'm careful and align everything right I shouldn't need any splints or splines to fill gaps. It's a bit tricky to get the cleats (thanks for clarifiying) to conform to the inside of the arch. I guess I can try the string through a tiny hole to pull it tight method, although I 'd prefer to figure a way just using strong magnets.
Thanks for your help.
jim

[Brian Evans]

I would cleat any of the cracks that terminate at the F-hole, just at the edge of the hole. I'm actually laminating a thin thin ply of cross grain wood or cloth completely around the F-hole, they just seem to want to generate cracks.

[Barry Daniels]

Really small cleats will fit against the arched top just fine. To place cleats at some distance from the soundhole/f-hole use a piece of a bent coat hanger wire that has a sharpened knife point on the end to stab into and hold the cleat during placement. Put some glue on the cleat and carefully place it using mirrors and internal lights to get it right. Hold it in place for a few minutes. Then you can let go of the coat hangar and rest your arms. After about 10 minutes, you can pull the coat hangar out. Obviously, do some dry runs to work out the moves. This technique gives a cleaner result than using the string though a hole method.

[Jim Hepler]

Thanks Barry. That sounds like a good way to do it. I'll give it a shot.

Brian, Next guitar I think I'll do something similar to what you describe. For this one I will be putting cleats at the f-holes as well as trying to get some further back along the cracks.

[Brian Evans]

The thing about gluing cracks on an archtop compared to a flat top is that the top is around twice as thick, so you have significantly more surface area in the glue joint. Makes your chances of a great result a lot better.

[Barry Daniels]

That's true, but the arched top and the lack of easy access through a smaller soundhole does present it's challenges.

[Jim Hepler]

I've got the guitar fixed up and back together now. Thanks a lot for the support and advice. I used hide glue, and a few cleats. The finish was shellac, so a bit of spot repair wasn't much of a problem. Had to fix the pickguard and resolder the controls to the jack. If you know where to look, you can still tell where some of the cracks were, but all in all, I'm pretty pleased.

[Beate Ritzert]

Nice job, i like it.

As long as it sounds well, this will contribute to the character of the guitar as it'll get older.

[Barry Daniels]

Looks good from here.

[David King]

That went quick! Congratulations!