Excessive soundboard "bellying," caused by string tension, can make an acoustic guitar difficult to play. It can even damage the top, bridge plate and bridge. Replacing the original bridge plate to correct the problem is a time–consuming job that can unfortunately reduce the value of a vintage instrument.
Respected luthier and restoration expert TJ Thompson developed a simple tool set, the Thompson Belly Reducer, that can fix soundboard bulging without permanent alterations to the top or bridge plate.
The Thompson Belly Reducer is a set of five precision-machined aluminum tools: convex flatteners for vintage–style belly and rectangular bridges, and concave flatteners for the three most common bridge plate shapes.
Aligning the cauls is now easier—magnets are embedded right into the tools. Magnetic cauls stay in place (hands free!) while installing and tightening the clamps.
They're designed for use with the bridge removed from the instrument, and are effective for bridge plates that have been installed with hide glue, aliphatic (yellow) glue, or PVA (white) glue. Simply match the appropriate flatteners to your guitar, heat them to 150 degrees F, dampen the wood on the bridge area and bridge plate, and clamp the flatteners into place for a day or so.
TJ Thompson recommends using three Ibex bridge clamps.