Official Luthiers Forum!

I'm no luthier but a hobby woodworker on a mission to rescue beaters into decent playable guitars that I can give away. I was given an old Yamaha dreadnought that sounds marvelous but had taken a hard hit on the back edge of its plywood back:




The edge of the plywood is smashed for about 6" along the edge. It doesn't show in the photos, but there's daylight between the plywood and side for a couple inches. My intent is to stabilize the damage and close it up cosmetically. My plan is:
1. Clamp the back to close and glue the cracked lining.
2. Fit a few small blocks at the repaired lining that extend 3/4 over the good plywood.
3. Dremel away the bad plywood to a clean edge and fill the hole with black epoxy.

Any comments or suggestions? Do I need to reinforce the epoxy fill?

sounds about right... you could fill the back with rosewood veneer to hide the damage but its generally not worth it to go all out on something like this and plywood is hard restore its structural integrity once broken.

_________________
Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

It's hard to say without seeing the instrument in person... and in my experience, plywood fixes are always a PITA. That said, I wouldn't dremel away the plywood - just "persuade" it the best you can back into shape. I would also hesitate to fill the gap with epoxy unless absolutely necessary. I would fill with more wood, if I could.
Anyway - good luck. I feel your pain.

_________________
"Act your age, not your shoe size" - Prince

Instead of workng the plywood you might consider replacing the whole back. The plastic purfing is gone, the binding is probably damaged too, for experience, you might just go all the way and replace it. More money, and time ...certainly.

I'm working on a guitar now that has a plywood top mostly as a test bed. I'm not really interested in building directly as I play much more than build or repair but I don't really like the way most guitars are setup and want to try a few things differently.

Most guys on this forum build for money, and there's nothing wrong with that. They see this as a lost cause and wouldn't waste there time. From their perspective they're right. I personally see this as a great move not just to help others but to improve your skills as a luthier.

You may contact one of our tonewood suppliers and they may have an orphan back that they could let go cheap.

I've played some Yamahas that were respectable sounding. In replacing the back you would likely improve the sound as well.

More work, More time, More fun!!

_________________
Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Actually, I had planned to use this one as a cadaver for experimenting and learning more about guitar repair. I made the mistake of stringing it up first with some almost new EJ-19s to see what it sounded like. I was completely unprepared for the quality of its sound. Even my wife came out of the kitchen asking if "Was THAT from the junker that lady dropped off this morning?" This one deserves a 2nd chance. I also ordered a Bridge Doctor to see if I can get the belly out and restore the action too. Replacing the back, though, is way over my pay grade.

Rather than using black epoxy, you could tint clear epoxy with pigments or alcohol based shoe dye to a color that would be a better match to the guitar. If you could find a piece of veneer that would be a good match you could dye it and embed it in the epoxy.
A decent repair could be made by rebuilding the plywood substrate and matching a piece of veneer into the existing back, staining and refinishing. Is the guitar worth doing a good repair on? Sure , why not! You aren't in it for the money, the cost of the materials is low, and you are by your own admission a hobbyist woodworker (which means you do this stuff for the challenge and the fun!). When you pull off an inconspicuous repair the guitar will not only sound good , but also look good.

If I can find a suitable piece of mahogany veneer, I'll try that. How would you go about rebuilding the plywood? I was going to bevel the good edge to build the epoxy fill but for a patch I'm not sure how to prep the good plywood edges.

If you really want to learn something with this cadaver, then rout the back off, remove the braces for reuse, repair the kerfed lining, and install a medium grade mahogany back. This is not ideal since you won't embed the braces in the kerfed lining. But no big deal. You will learn a lot doing this.

Most folks will not bother reparing a plywood guitar. It's normally not worth the effort ($$).

Mike