Bandurrias are tres cool, but they've got some distinct issues due to 12-string tension and the Spanish heel design. This one is one I've slacked-off on for a long time until the proper repair solution gelled for me -- as the neck joint had caved into the top and sprung the side-seams. Of course, a few layers of old repairs accompanied this condition, too.
You can see how tension and a caved neck joint have spread the sides right out from under the top.
The soundboard below the fretboard is almost entirely detached from the rest of the soundboard and has a "fault" running along both sides of it. It's that usual "crunching" effect that you see pretty often around soundholes.
To be able to get the instrument into a condition where I can start repairing it, I've run a threaded rod inside with some couplers that allow me to expand it.
When I do so, I let it push at the top of the neck block and the the top of the endblock -- thus forcing the sunken neck geometry back to a more-original placement. On some instruments I've installed a rod like this through the body and anchored to the endblock with a threaded recepticle -- thus maintaining reverse tension to counteract the usual neck-sink problems of a poorly-designed or decrepit instrument.
That sort of modification works beautifully, boosts sustain, and frees the top up to vibrate more effectively, too, but it's not for everyone, of course. Haynes/Tilton did a similar design in the 1860s, the Larsons were using the same idea in the 20s/30s and Epiphone was doing it in the 30s/40s as well. Fender then re-hashed it (a bit heavy-handed) in the 60s. It never catches-on but it can work without killing the sound if done right.
Fortunately for me, the owner of this instrument has given me the go-ahead to install one of these "for good" (and with hidden access at the tailpiece) once I'm done with repairs. They can always be taken-out later, but on an instrument like this where structural integrity is already compromised, it's a good solution to an almost-wallhanger.
The trouble with this instrument is that to get enough structural rigidity to resist the damaged neck area's creep and push on the seams, I'd need to take the back, sides, and top off and replace all the kerfing and do a major overhaul of the bracing around the neckblock. Those areas just happen to be weak in the build, despite the fact that the instrument itself has good workmanship, otherwise.
Anyhow, with the neck tensioned-up from inside you can see how the sides have pulled right back to where they were when this was built. There's not even a gap at all anymore. The next step of the repair was de-tensioning the rod, shooting in some glue, and re-tensioning and taping-off to hold it until I could get a clamp on it.
Before that clamp, though, I set to work fixing the failing neck-block area...
Here's a new cedar patch to install right where the joint failed worst and separated the soundboard from itself under the fretboard extension.
I'll rough-cut it, glue it up, and then sand it with a barrel on my Dremel tool to make it look nicer after it's all set.
To this layer I added a longer, wider layer that I glued over some thin "tonebar" or "stiffener" braces to either side of the soundhole. Between the two of these new cleat/patches, twist, distortion, and sag at the neck block should be greatly reduced even without a "rod install."
Here's a mirror shot of one of the braces that follows the grain of the soundboard to either side of the soundhole. My first "patch" layer is level with these two braces and then my second "patch" layer straddles my first "patch" and continues over the two braces. This should add a lot of strength and help bulk-up the main under-fretboard brace that seems to not be handling much structural work anymore.
It's a bad shot, but you can see that it's glued-up over it, now. Excess glue will be wiped, of course.
The end job needs a lot of awkward clamps, huh? Note the massive one that's correcting the "fault line" to the treble side of the fretboard extension!
Today was certainly "crazy mandolin day." I did structural work on 5 of them, the bandurria, and a Gretsch "Camp Uke" knockoff.