Hi, Newport Boy. You mentioned a guitar with a crack in the back. When my guitar got cracked, I asked my brother, the expert what to do. He asked if the cracks went over a glued joint. They did not. Then he told me to drill holes in the ends of the cracks to keep them from propagating. Make the holes as small as possible to do the job.

My cracks were simply folliwing the delicate grain lines of my wood, so we used the smallest drill bit there is - a sewing machine needle. I bought some wood putty and filled them in, but I didn't have to.

The cracks have not grown in 20 years.