Guitars often are more than musical instruments. They are treated with respect, care is taken to shield them from damage, and steps are taken to ensure their protection. However, some guitars are prone to crack due to stress placed upon them by the whammy bar. Sometimes a guitar gets dropped, falls off the stand or a guitarist might crack it in the throws of a performance. Whatever the reason, almost any crack in a guitar body can be fixed with a drill, some glue and a dowel.
Examine the guitar to see if the crack is visible on the back. Examine to see if the crack runs under the pick guard; if it does, take off the pick guard. Fixing the crack from the back or under the pick guard is the ideal place to start simply because of cosmetics. It's easier to hide on the back or under the pick guard.
Decide which location will be the easiest to hide and start there. If you have to start on the front in the open, that's OK. Using the cordless drill with the 3/8-inch Forstner bit (a Forstner bit is a special bit that cuts very clean), begin drilling vertically 3/4 inch beside the crack. When you have penetrated the surface of the guitar to approximately 1/16 inch, angle the bit and drill diagonally through the crack to a depth of approximately 1 1/2 inches.
Clean out the hole. Glue and tap the dowel into the hole, leaving 1/16 inch protruding out the hole. Let dry overnight.
Cut off the end of the dowel with a sharp chisel. Spot sand the dowel flush with 100-grit paper. Switch to 400-grit wet sandpaper and continue sanding until blended. Stain the end of the dowel. Using a small brush, apply lacquer to the sanded area. If the crack is visible, apply a thin line of lacquer to the visible portions of the crack to seal it. Let dry overnight. Rub down the sanded area with rubbing compound, then finish off with polishing compound
You can get Forstner bits at your local hardware or home improvement store.