When an acoustic guitar's top bows out, it greatly affects the sound quality of the instrument. Unlike solid body guitars, which rely exclusively on electronics, acoustic guitars get their sound from the body itself. While the back may be flat or curved, the top must remain flat, otherwise the sound is drastically altered. A guitar top may bow because of humidity or improper bridge and string tension. Though fixing a bowed guitar top is not difficult, it will take several days or even weeks.
Lay your guitar on a flat surface such as a table or bench inside your home. Do not place the instrument in a room or space that is not temperature controlled.
Remove the strings by unwinding the tuning pegs.
Get two round plastic container tops, such as food storage plastic ware or empty butter tubs.
Cut out a shallow U shape in one round plastic top to fit around the base of the neck where the fret board is affixed to the top of the guitar.
Place the uncut round plastic top inside the soundhole of the guitar.
Wet a cloth or sponge and set it directly on the plastic top inside the soundhole; the plastic top will protect the back of the guitar from absorbing the moisture.
Fit the cut top over the soundhole, abutting the cut notch tightly against the base of the neck. The plastic top should should "seal" the soundhole.
Stretch a large elastic band over the guitar to hold the plastic top over the soundhole. Monitor the guitar and wet the cloth or sponge every couple of days. The bow in the top will begin to flatten in about two to three days.
Leave the cloth or sponge and plastic tops in place until the bow entirely disappears from the guitar.