How to Deal with Bulging Soundboards and Moldy Cases

Top bulging can affect long-term playability, especially on flattop guitars. And moldy cases can affect the smell of a guitar, even when it’s left to air out for a while. Here’s some advice on how to handle both bulging soundboards and moldy guitar cases.

Q: How do I overcome the issue of guitar tops bulging upwards, thus raising the string action, no matter how many bridge or truss-rod adjustments are applied? 
—Seán Ó hArtaigh 

A: Top bulging, or distortion, is without question the most common culprit behind long-term playability issues on flattop guitars. In both modern and vintage instruments, the constant string pull eventually causes the top to deflect forward, raising the action beyond what is comfortable. The short-term fix is to cut the string saddle lower, which can help, but this lowers the effective break angle of the string, which in turn reduces the strings’ leverage on the top. If overdone, this can result in diminished volume and response.

The proper repair for a bulging top is a neck reset, which involves disassembling the neck-to-body joint, usually using steam; recutting the neck angle; and regluing the neck in its new position. This repair is commonplace, and most competent luthiers have some experience doing it. But it is still a pretty invasive job, and can be expensive.

In recent years, a new approach has emerged, largely led by expert repairman TJ Thompson. This technique involves softening and reshaping the glue joint between the top and bridge plate, which is the thin but hugely important sliver of hardwood inside the guitar underneath the bridge. When done properly, a considerable amount of top bulge can be remedied without taking apart the neck joint, which is a great time saver and spares the guitar considerable risk.

There are products on the market that are designed to counteract top bulging. The most well-known is a sort of truss system that attaches under the bridge and transfers the string pressure to the guitar’s tail block. Some manufacturers have even built this system into brand-new guitars as a hedge against future issues. Although it may be possible to design a guitar to perform effectively with such a system in place, most are not built to have their tops constrained in such a fashion, so it is quite possible that installation of such a device would have a noticeable impact on the tone and response of the instrument. That said, if it made the difference between a playable guitar and a wall hanger, it might still be an appropriate choice in certain cases. For guitars with any kind of vintage pedigree or value, I would hope that a good repairperson would advise the owner to pursue a more thorough solution such as a neck reset.

In closing, this is a great opportunity to remind all responsible guitar owners to keep careful tabs on the humidity in their homes or guitar rooms. An overly damp guitar will often exhibit considerable bulging in the top, whereas an excessively dry guitar top will flatten out or even become concave in some cases—and will often crack shortly thereafter. If you are experiencing high action and bulging tops on multiple guitars, the best place to start would be by monitoring your environment, and dehumidifying if your levels are too high. A good safe midpoint would be 45% relative humidity at normal room temperature (about 70 degrees Fahrenheit). Small digital hygrometers are cheap and widely available; I recommend buying several and storing them in a few different places near your guitar storage. In many cases, climate control is all that is necessary to restore guitars to proper playability.

Q: I bought a used guitar whose case has developed a strong moldy smell. I’ve tried leaving the case out in the sunshine for several days, washing it with vinegar, etc., and the smell always returns. I’m worried that if I replace the case, the guitar will carry mold spores to the new case and I’ll eventually have the same problem. Do you have any suggestions? 
—Charles Boyd

A: I’ve had several clients come to me with this specific issue. To be frank, there is nothing I know of that can completely remove the mold smell from a case. Nice hardshell cases are a good investment, and usually worth the money, so I’d suggest simply buying a new case for the guitar and getting rid of the old one.

If the guitar itself is suffering from the moldy smell, it may need a good cleaning. This should be done with care, as some common household products can cause issues with lacquer finishes. I’d suggest starting by removing the strings, taking care to set aside the saddle and bridge pins in a safe place. Clean all the finished/polished surfaces first with a damp rag—not wet enough to drip, but just damp. A drop of liquid dish soap can be helpful. Clean with small, circular strokes, and pay attention to make sure you are not leaving streaks or scratches in the finish. Avoid cleaners with any kind of solvent, alcohol or otherwise, as these can damage lacquer. If you really need the big guns, naphtha (aka lighter fluid) is about the safest solvent I know for most common guitar finishes.

The harder area with moldy guitars is the interior. I’ve seen more than a few with visible green mold inside the box itself. Water/soap and other cleaners can leave stains on the unfinished wood inside a guitar, so I usually use alcohol on a paper towel to quickly wipe down the interior, as far as can easily be reached through the hole. Don’t overdo it!

In some cases, I have finished by polishing the guitar with a very mild abrasive compound such as Novus 2 scratch remover. The very fine grit of this compound helps to scrub off any final mold that may be on the surface of the instrument. It also usually leaves a nice shine, though you may have to scrub through a few layers of grime before it starts to look glossy.

This article originally appeared in the March/April 2023 issue of Acoustic Guitar magazine.