When my guitar had a mishap and got some fine cracks in the soundboard, I called my brother the genius. (This was several years ago.) He said, "The big question is, do any of the cracks go over a glued joint?"

My answer was no. That was good. He said we should drill holes in the soundboard at the end of the cracks, matching the size of the crack to the size of the hole. The round opening will prevent the crack from going further. Then I could fill the holes if I desired, though it is not strictly necessary.

So we put a sewing machine needle in the drill, and made small holes at the end of the small cracks. I filled the holes with putty from the hardware store. The instrument's been fine ever since.

If I owned a guitar with a crack below the soundboard, I would drill the holes before the crack advances any further.