Repairing a guitar that sat down too hard
A neighbor, having heard that I have had some success with broken guitars, asked if I could do anything with his Yamaha electrified folk guitar. It appeared that it had perhaps been dropped "on its butt", because the side was fine, but there was a crushed area of the back, down near the strap button, where the side had apparently pushed in then bounced back out. (I should have taken a photo.) Back wood was missing for up to 1/4", over a span of about 4", with radiating cracks in the back finish for several inches from the point of impact. That part of the back was elevated a little, not matching the sides at the corner.
The first step was to clear out bits of loose, broken wood, then verify that the back could be glued into place at the sides. I used a toothpick to work some Titebond glue into the broken corner, then put a slab of stiff plastic, wrapped in teflon sheet, over the wound, and applied three big trigger clamps. (The slab spreads the pressure from the clamps, and the teflon sheet keeps the glue from sticking to the clamp system.)
After the glue had cured for about 22 hours, I mixed up some carpenter's wood filler and packed it into the remaining holes where the back wood had shattered. (I had to mix a little water into the old filler to get it soft enough to penetrate and stick.)
After the wood filler had cured for a day on two, I smoothed it off with a file, and then with sandpaper. (I could have / should have added more wood filler to raise the concave area, so the file could smooth it, but that didn't seem necessary.) Then, I used acrylic artist paint (color "licorice") to paint over the tan wood filler. It was a pretty good match for the almost-black wood of the guitar.
After the paint dried, I coated it with clear lacquer. Lacquer uses some pretty nasty solvents: acetone, methyl-ethyl ketone, and maybe some alcohols, so I only do that outdoors. The benefit is that a new coat of lacquer partially dissolves the old lacquer finish (or, so I'm told), so some problems can be fixed by adding more, without stripping the old finish off of fragile wood.
After the lacquer dried, I buffed out some brush marks, and put on a little automotive paste wax.
I'm not sure exactly what the owner was expecting, but he was delighted with the results. "Woahh, man!" I said it was a gift, but accepted a $20 gift in return. He is a neighbor, after all, and at this point, goodwill is worth more to me than cash.