Martin D-1: Help with a Crack

[Robbie Collins]

I just received this guitar and it has a nasty crack on the right side of the back. Owner says his girlfriend stuck it in the attic, but it looks to me like it may have experienced trauma as well. At the center of the crack, and as seen in the photo, the upper part is 1/16" proud of the lower. 

Looking inside, the 1st brace is still attached at the end but detached an inch on each side of the crack. The end of the 2nd brace is broken and missing but is solidly attached over the crack.

My plan is first put a humidifier inside the case and see if I can get the crack to close, then make 4 molded magnetic clamps (http://frets.com/FretsPages/Luthier/HighTechDept/Magnets/MoldMagCau...) and use them to level the edges before clamping the crack shut. When the crack is solid then I'd reglue the braces. Q: is it important to replace the end of the 2nd brace?

I'm thinking that if I coat the clamp form with a thin layer of vaseline that I might be able to reuse the form. Has anyone tried this?

I'm also thinking that because the crack is so displaced, and because it is so near the side where more torque will be required, that neither HHG nor CA will withstand the stress and epoxy might be better.

Lastly, in the first photo you can see that the tortoise binding has come loose over the waist. Ditto for the other side. The upper binding is fine.  I plan on using a hair dryer to soften and expand it and tape it into place so it retains its shape after cooling.

Any and all comments and suggestions appreciated. Thanks, Robbie.

[Jelle van Tilburg]

Hello Robbie,

Personally I would try to get the crack level, glued en cleated as neatly as possible..and then step away from the guitar.

Those catalyzed satin finishes are a nightmare to touch up (to say the least). And you might end up with something that looks far worse. (And in my opinion you would hardly call this a museum piece, that would justify extensive touch up.)

If you really insist in doing any touch up, maybe just something like a subtle bit of stain and a light coat of shellac brushed on with a thin brush. (mind you, do some tests on scrap first, as the shellac might weaken the stain/change the colour) 

I would like to know how other people feel about this (would you call a repair like this finished without any touch up?)

[Ned Knepp]

Have you tried using something steel with the magnets instead of other magnets? It may not hold quite as well but you may be able to double up the magnets on the outside to compensate.

A nice, thin bladed knife may work as well as the chisel if  you can't keep an edge. I really think that sharpness may be more important than which tool you use. I'm still not sure that completely removing the brace isn't the better idea since it looks like it's half way out already but you can actually see the  guitar, I can't. 

I've patched up a couple of instruments with CA on the "impregnable" poly a time of too.  It works as far as sticking to the finish but it doesn't blend. On one, a soprano Ukulele (small top), I decided to strip the top and refinish because the crack also chipped some of the original finish which left me with some wider fills. There was just no way to make it look good. The other filled a fine crack that was acceptable because it was a fine crack.  

Flat finishes are notorious for being hard to blend repairs on but if this is a catalyzed poly finish and nothing works as a solvent, it may be to your advantage to flatten the repair a bit. I've use, on the advice of  some of our fellow posters, some very fine steel wool to lightly abrade the repair to flatten it a bit. It's never going to be flat like the sprayed finish simply because it's really just a very light sanding and leaves "grain" in the finish because of it. I think it's better than a bright, shiny streak but it's not going to disappear either. 

BTW have you tested the finish in a out of view/way spot with any solvents to see if it can be melted? I''m confused about what Martin uses for finish but I believe there is a chance that it's nitro. If you can find a solvent everything will be MUCH easier. 

[Robbie Collins]

UPDATE:

I cannot get the surfaces to align on the z-axis (vertically). I tried using three screw jacks spaced along the inside seam of the crack. The two sides get close but not level. More torque on the screw jacks and the back would crack behind the jacks.

I looked at the crack under magnification but found no debris. I slide in a piece of 220 sandpaper and gently took off the sharp edges. 

I also tried using  5/8"x 5/8" magnets in three locations with two on top and one inside at each location for a total of 9. Same result  - close but no cigar. 

I'm thinking that the side of the crack closest to the edge is "sprung." I checked the kerfing along that edge and it seems tight. I can't see anything jammed between the back and the kerfing that is holding that joint open.

It takes so much torque to push the edge even I believe that if I got the surfaces level, clamped and glued the seam wouldn't hold because of the internal stress. What to do?

All ideas and suggestions appreciated!

[nathan clark]

I confess that I haven't read every word of this thread so forgive me if I overlooked something.

Can you remove some of the binding and pop the seam in that damaged area?  That should give you clamping access.  

It's on the back of a cheap guitar so it would be appropriate to patch in a small piece of matching binding to make up for the shrinkage.  That would spare you the extra time of removing the binding all the way to the joint at the heel.

If you are finicky about pulling the binding then it will look fine without a finish touchup, which is basically impossible with this kind of finish without overs-praying a good chunk of the guitar.

[David Farmer]

 The jacks should be just snugged up and the pressure is applied w/a clamp on the outside of the body. Flat cauls on the inside and outside of the top and back. If you use clear polycarb plastic you can kind of see how it looks in a test clamp.  The jacks don't apply the pressure, the clamp on the outside does. If everything is lined up carefully you should be able to just about crush the wood level. Does that make sense?

[Robbie Collins]

David ... you made me slap my head and yell "duh!" ;-)   I am using a clear acrylic flat caul on the outside. I'll try it as you described. I still worry that the joint will be under a lot of torque.

[Robbie Collins]

Thanks for the ideas, Nathan.  Patching the binding is a good idea. I've ordered a Black + Decker variable temp heat gun and will try to stretch the binding before I resort to cutting it. I read a comment (I think from Hesh) that the trick is to set the heat gun below the temp that bubbles the finish. Any idea what that temp would be for a Martin catalyzed lacquer?

[Jelle van Tilburg]

Robbie, 

Just some quick thoughts.

What happens if you excert pressure with wider flexible cauls (instead of solely pushing up with the smaller screw jacks from the inside)

And you're sure you're not fighting one of the braces still partially glued along the cracked area right? (because that of course would also affect alignment)

Good luck!

[Robbie Collins]

Hi Jelle ... Do you mean flexible cauls on the inside? I ran a palette knife under the braces to detach them over the crack so that's not a factor.

[Jelle van Tilburg]

Hello Robbie, 

I'm sorry I don't have a definitive answer for that. I personally would try whatever way and whatever caul combination needed to get things level/lined up. 

I have used different combinations and lengths of thin sheet plastic/acrylic and some heavier flexibel and non-flexibel cauls (inside and outside) for different glue ups

I couldn't find any pictures of a glue up with the cauls in action. But I've added a picture of the cauls to give you an idea