A Primer on DIY Finish Touchup for Acoustic Guitar Repairs

When working on an acoustic guitar, how can you determine what type of finish was originally used? And how can you best match it to complete the repair?

Q: I am working on a Martin with a couple of cracks, which I fixed by adding spruce splints. I would like to touch up the thin satin finish. How do I determine if it is polyurethane or lacquer, and what is the best way to proceed? 
—Antonio Rojas Flor

A: Congratulations—you have discovered a door to an alternate universe. Unfortunately, it is one full of potential pitfalls and disappointments. Touchup work is a dark art that can take many years to master. As with all such refined skill sets, I would encourage you first to read as much as you can, and to practice on some unimportant material before working on your Martin.

If you are trying to match the top color, I would recommend sealing the repairs with shellac first. Be sure to use dewaxed shellac, which will not cause adhesion issues with any subsequent finish products. A thin coat is enough to seal the repair and allow color layering, without the stain bleeding deeply into the bare wood of the splint. Use a fine-tip brush and masking tape to keep the shellac off the surrounding finish as much as possible.

I have had very good results using artist brush pens for color touchup—the Tombow brand is my favorite. These come in a very wide range of wood tones with a very fine tip for precision work. The water base also means they can be finessed or removed, if necessary, with a damp Q-Tip.

Martin has used many different finish products over the years, so it’s hard to know whether your guitar is lacquer or some other more modern product. You can test for lacquer by dabbing some acetone on an inconspicuous area to see if the finish softens. I have used the area underneath the tuning machines for this test in the past), but in my general touchup work, I often proceed the same way for either finish. Satin finishes get their texture from flattening agents mixed into the product, and only look correct when they are directly out of the spray gun—there is no easy way to buff or sand a finish to a convincing satin texture. 

Fortunately, Mohawk Finishes makes a spray can lacquer called Perfect Blend, which is available in a satin sheen that almost perfectly matches the Martin finish. Look for the Matte 19-23 Sheen or Satin 35-40 Sheen, depending on which you think better describes the finish on your guitar. The wizards who designed these spray cans have managed to make a product that matches the factory finish almost perfectly, which is a minor miracle.

As I said, touchup can be a maddening exercise, but those who excel at it are among the most impressive craftspeople I know. For an example of the art at its absolute peak, look up Iris Carr, the brilliant violin repairperson. Her touchup work borders on magic, and shows what is possible with patience, dedication, and decades of experience. Best of luck!