Acoustic guitars are largely made of wood and react to their environment. Solid top acoustics are more sensitive than laminate top guitars, and will require more care. Cracks in the body of your solid top acoustic guitar are often cause by physical damage to the guitar or shrinkage due to a lack of humidity. Depending on the severity and location of the crack, a repair is as simple as stabilizing the wood and applying glue to secure it.
Remove the guitar from any sources of heat, such as radiators or furnaces, to prevent the wood from drying out and worsening the crack.
Place the guitar on a flat surface.
Determine the location and severity of the damage. Simple cracks and splinters that do not affect sound or playability can be fixed at home.
Using a utility knife, gently scrape away any splinters or loose wood from the crack.
Reach inside the sound hole and gently press the cracked wood upward toward your body so that the wood is level again with the rest of the guitar.
Using your other hand, slowly insert glue into the crack until the length of the crack contains glue. Wipe away excess glue with a wet cloth and repeat this step until enough glue has been placed in the crack to be level with the surface of the guitar.
Wait for the glue to dry. Depending on the type of glue, this can be up to 24 hours.
Apply glue to one side of the cleat. Cleats are thin, flat pieces of wood usually made of spruce or the material of your guitar. With the cleat in one hand, reach into the sound hole so that your hand is on the inside of the guitar, underneath the location of the crack.
Press the glue-side of the cleat to the upper interior of the inside of your guitar, directly underneath the crack. Make sure the grain of the cleat is placed perpendicular to the grain of the wood on your guitar. This further stabilizes the crack.
If string tension adds further stress to the damaged area, unwind the strings until you are able to repair the guitar.
Don't attempt to fix an expensive instrument or one with significant damage, as home repair can greatly decrease its value or cause more extensive repair.