Acoustic guitar belly repair

Over the years I’ve come to realize that there’s two major philosophies that one can follow when building an acoustic guitar: it can either be built to last, or it can be built to sound good – but it can’t be both. Guitars that are built to last tend to utilize heavy internal bracing and thicker tops to resist the constant pull of the strings, and generally don’t sound very good because they are so stiff. Guitars that sound really good tend to have very light bracing and thinner tops, but the constant pressure of the strings can cause the guitar to slowly collapse upon itself. Guitar makers over the years have always treaded the fine line between an instrument that sounds good but won’t completely fall apart, and many have produced some fantastic instruments. However finely built, however, every acoustic guitar is prone to having issues directly related to the constant pull of the strings, and one of the most common issues is a bellied or warped top.

A standard steel string acoustic, strung up with a set of 12-54 strings, is constantly under about 75kg (160lbs) of pull. Eventually, many guitars succumb to the pressure and develop a top with a bulge at the bridge. Oftentimes this is coupled with the bridge lifting as well. A bellied top can cause high action, bad intonation, and a weak sound. This guitar came into the shop last week, with a severely bellied top and a lifting bridge:

bellied top

Now, an acoustic guitar’s top shouldn’t be dead flat (they are arched, typically with a 4.5m – 6m radius), but this one was severely arched. This one was definitely in need of a debelly job.

To fix a bellied top, the bridge first needs to be removed. To do this, the glue must be heated and softened and then the bridge is cut away. This particular guitar was glued together with epoxy, which doesn’t really respond to heat, so I anticipated this to be a doozy. I first made a heat shield with aluminum foil and cardboard to protect the guitar’s top, then set a heat lamp on some blocks about 30cm from the top:

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You really have to be careful when applying heat to an instrument, and make absolutely sure that the guitar is protected – I know a guy who once forgot to put down heat shields, and the guitar caught on fire! Yikes!

After a few minutes under the heat lamp, it was time to go to work on that epoxy. I knew that it probably wouldn’t come off without a fight, but fortunately I had a fancy new bridge knife┬áthat worked like a charm. I barely broke a sweat getting this off, and usually removing an epoxied bridge is a serious chore.

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With the bridge off, now it was time to address the warped top. To do this I used a couple of Thompson Belly Reducer cauls, which are shaped to match the bridge footprint and internal bridge plate, and are curved with mating concave and convex arches. These are heated and then clamped in place, which will press the belly out of the top.

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This guitar didn’t flatten out perfectly the first time, but on a second attempt it flattened out the rest of the way. When dealing with wood and heat, I’m oftentimes repeating the same procedure several times to get the result I want. It’s got to be done right, even if it requires extra work!

While waiting for the heated cauls to work their magic, I used a chisel and razor blade to remove the excess wood on the bottom of the bridge that lifted when removing the bridge. It’s rare they come off clean, especially when they’ve been epoxied on, and all that wood and glue needs to come off.

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Once the top had flattened out two days later, it was time to mate the bridge back to the top. The bridge had warped along with the top, and so now it was misshapen. To shape it to match the top, I applied some 80 grit sandpaper over the bridge’s footprint, and sanded the bottom of the bridge down to where it sat perfectly.

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Finally, I glued the bridge down with Titebond III wood glue (the same stuff Fodera uses to build $30k basses) and clamped it in place. I waxed two of the bridge pins and used them as locators, so that the bridge would be positioned perfectly and wouldn’t slip.

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Pow! The belly is removed, the bridge is back to it’s proper seating, and is as solid as the day it was new. This repair should last another several decades. Great success!

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