Sometimes acoustic guitars develop a bit of a belly. Over time the string tension pulls on the bridge, angling it forward and warping the top, which in turn raises the action and causes other structural problems down the line. Here’s a particularly bad case:
To fix this we have to remove the bridge and address the top directly. We have a cork board that we’ve cut a bridge shaped hole into, so we set that on the guitar to protect the top, and then heat the bridge with a 75 watt lamp. This melts the glue holding the bridge to the top, allowing us to slide knives into the glue joint and lift the bridge:
With the bridge removed, we can now start work on fixing the top.
We’ve got a special tool for this job: the Thompson Belly Reducer – a set of five precision-machined aluminum tools: convex cauls that go on the top and concave cauls that go inside the guitar on the bridge plate. These are heated, put in place on the top and on the bridge plate, then clamped. The trick is that the heat softens the glue between the bridge plate and the top, which allows the glue joint to slip while the top is being pressed back into shape, and when the glue cools it holds the top in it’s proper shape. Here’s how we do it:
We heat the aluminum cauls in boiling water, just hot enough to melt the glue, but not so hot that they mess up the finish on the guitar’s top. Gloves are essential here – safety first!
MMMmmm… aluminum cauls…
Once the cauls are hot enough, we quickly pull them out of the water with tongs, set them in place, and clamp them down. The convex and concave cauls reshape the top, pressing it flat.
And that’s it! The heat and pressure from the cauls will do the rest – we just wait for the cauls to cool and the glue to set. Then we put the bridge back on, and the top will be as good as new.