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I'd like to build a nylon-string guitar next, so I bought one to play, measure, etc. It's a low-$ Cordoba C5 needing repair -- a neck crack from freight damage. At the nut, the crack starts as glue-separation between the fretboard and neck, then runs in that ~same plane on one side, and a little into the wood on the other. These flash photos pop it:


Gently wedging in my thumbnails at the nut, I can open a 1/32" gap. The first 1/2" of crack appears to be just glue separation; after that I can't tell. I'm not looking for an ideal repair; just serviceable. Some questions--
- I know the usual procedure of working thinned wood or hide glue into a crack. Would that be effective enough to rebond the fretboard to the neck (I'm guessing PVA from the factory)?
- What finish would be good to keep the factory finish from chipping off?
All tips appreciated!
David

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David Malicky

It doesn't look like a scarfed neck joint, so my guess would be 1) bad glue, 2) the truss rod was inserted incorrectly, or 3) adjusted poorly, causing the FB to separate. When you spread the crack, does the neck wood separate with it? In the photo it looks like it may just be the finish.

Since this is a nylon string and I believe the tension of the strings will actually compress the "damage" I would do the complete repair using CA -- crack and finish. I believe you will be able to make a very strong invisible repair. If this does not work you can go into major mode. $.02

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Ken Cierp

http://www.kennethmichaelguitars.com/

Thanks, Ken and Glenn!

Glenn, I also thought about the truss-rod as a cause, but in this case it appears to be due to mishandling/shipping damage. There is a nice ding on the top corner of the headstock to go with it! Yes, the wood does open up slightly when I 'pry' up the fretboard.

Ken, Thanks! Yes, I agree the string tension will help keep it closed. I haven't done much finish repair (or I have been doing it the hard way up to now)... I just learned today about the CA method via the archives and youtube. Slick! Maybe I can feed it from the nut and get it to wick to the finish surface from the inside. But the regular way looks easy, too.

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David Malicky

I'd go with Ken's C/A repair. Putting PVA glue into a PVA glued joint makes a really lousy joint repair. Most of these kind of shipping pops come from not enough padding UNDER the peghead in addition to OVER. Mandolins are very prone to this unless padded well. UPS once told me that you have to figure the ability to withstand a 4' drop. That is their conveyor system. Of course the strings should not be under much tension when shipped...
Since when do classicals have truss rods?

Thanks, Haans, that's interesting. Good to hear that CA is better than PVA for this joint -- I was wondering about that. Cordoba started using truss rods some years ago, but I'm not sure exactly when.

One question on the type of CA: I know the thin is best for the finish and to seep deep in the crack, but is it the right choice near the nut in case there are gaps? I was thinking to drip thin in first, then medium last for better strength at the start of the crack.

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David Malicky

Good point on the truss rod question

These are my personal opinions regarding using c/a to reattach the fingerboard. I have never done this repair, but I have used c/a extensively on other, non-lutherie, projects.

A bit of caution on using thin c/a followed by thick c/a: Thin c/a sets lots faster than thick c/a. Thin c/a cannot be used to fill gaps. That job is left to the thicker versions of the adhesive. My experience is, the thicker it is, the slower it sets compared to something thinner. Also thin c/a has very low surface tension, it runs all over everywhere, so immense care is appropriate when using it. And you need to use very little of it. Have your clamps immediately at hand, having test-run the process without glue. You have less than 30 seconds, sometimes much less, to get a joint clamped up when using thin c/a. And if you can, leave the clamps on for a good long while, like an hour.

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Peter Havriluk

Thanks, Mike and Peter. About the truss rod as a cause -- the neck has normal relief and the crack is closed. If the truss rod caused it and was left tight, I'd expect the crack to be open; or if turned the other way, I'd expect back bow. Anything is possible, of course, but with the headstock ding, shipping damage seems most likely.

Peter, yes, that's my experience with CA, too. Good points on the runnyness, dry run, and cure time differences. I will mask the neck and guitar, except just around the cracks. Maybe tape over the cracks to keep the CA from leaking out, but the CA would bond to the tape, and removing it may pull off finish chips. For cure time, my BSI thin CA says 1-3 seconds! The crack does seem fully closed, so I could try just the thin, clamp, cure, and test. If it fails, then go thick.

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David Malicky

An update and lessons-learned:

It is fixed, strung up 4 days, and holding fine. I only used the BSI thin CA. I did use clear tape on the crack to keep CA from leaking out; that worked, but it may have led to another problem (described below). Feeding the thin CA from the nut, it did penetrate all the way down and throughout the crack, adhering to the tape.

Most of the repair is indeed close to invisible (thanks, Ken), but I have a few lighter-colored tiny inclusions/spots. I'm guessing these are either flushed-out PVA, unstained wood bits, or some adhesive tape residue that transferred in. So, it's not a professional repair (I could dig them out, stain, and redo), but it's plenty good for my use and I'm happy for my first of this type.

I also covered the neck in paper and tape, and put the body in a plastic bag -- thanks, Peter. But I didn't cover the headstock and got 1 dot of CA near the nut (sanded and buffed now). My biggest mistake was holding the guitar upright while feeding in the thin CA. It's hard to judge how much CA is going in, and it ran all the way down the truss-rod channel, around the rod opening, out the soundhole, and onto the TOP! Got a nice blotch right under the soundhole, with the protective plastic bag ironically adhered to it! It's sanded, buffed, like new, and a comedy now, but wow was that an Ooops! Some lessons for this type of repair:
- Hold the guitar horizontal. Surface tension should be plenty to pull the CA in.
- Cover every square millimeter of the guitar, but probably don't tape the crack itself.
- Feed the CA on the left and right sides of the nut/crack-opening; avoid the middle near the truss-rod slot.
- Maybe use StewMac's thin CA rather than BSI's. BSI's says it is 1-3 second cure so probably extra runny (but extra-penetrating). SM is 3-5 second.

Thanks again to everyone for all of the help!

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David Malicky

Glad to hear CA repair worked out for you -- "Hot Stuff" in all viscosities is my CA of choice and always has been. Those of us into RC airplanes were introduced to it many years ago.

There two great reasons for using thin CA for crack repairs (and it is stronger than most believe) first is the capillary action which draws the glue deep and all along the crack line even when its closed tight to begin. And perhaps even more important is it's ability to glue to glue. Which as pointed out is something that PVA's cannot do and provide the stronger than wood feature of the original joint bond.

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Ken Cierp

http://www.kennethmichaelguitars.com/