One of the greatest enemies of an acoustic guitar is environment. In Minnesota where I live, it is particularly important to protect your guitar during the long, cold, and DRY winter. The ideal relative humidity for an acoustic guitar is in the range of 45-55%. To keep a guitar in this range, you should always store your guitar in a case with a humidifier. Failure to do so can result in the relative humidity of the guitar dropping below 45% and may ultimately result in a crack.
While prevention is the best strategy, we’re going to talk through how to fix a crack if it does occur on your instrument.
A cracked guitar top is one of the most common problems we see with older guitars. It can happen to high end and low end guitars alike. As previously mentioned, cracks are almost always caused by improper storing of guitars, especially in climates that have very humid summers and very dry winters, and should serve as a reminder to always humidify your guitar. (Pro Tip: We recommend the Dampit Guitar Humidifier. It is very reasonably priced and works great.) A good rule to go by is to get a humidifier that completely covers the sound hole.
This post is intended to walk you through the steps of fixing a crack if one occurs on your instrument.
Materials needed for this repair:
- Large Clamp(s)
- Wood Glue
- Small Suction Cup
- Razor Blade
- Sand Paper (high grit 1000+)
- Mineral Oil
- Shims
- Painters Tape
This project began when a friend of mine had to find some space to accommodate a new child. The guitar room was becoming a nursery. And he was given strict orders to clear out all non-essential guitars. Quite a task. And this guitar did not make the cut. It was a Martin 000M Mahogany Auditorium. A beautiful guitar. The only problem was a small crack on the face of the guitar.
I asked how it was stored and he said it was never humidified. Easy to see the culprit behind this crack. We live in Minnesota and the summers are hot and humid, the winters are very cold and dry. Worst case scenario for long term guitar storage – if you don’t manage the humidity properly.
We were lucky as this guitar came with a natural matte finish, which is the easiest finish to match after repairs. The steps to repairing this problem are pretty straight forward on the surface, but do take some practice to master. You have to get glue in the entire length of the crack which takes some patience and the use of a small suction cup. After that you clamp the crack shut, clean the surface, and allow it to dry. Then you make a cleat which goes on the inside of the guitar under the crack. It is very important to ensure the grain of the cleat is perpendicular to the grain of the guitar. If you do this properly, the guitar will never crack in the same spot ever again. Then you clean the surface of the guitar and apply the oil. Sand and repeat as needed to match the finish. Restring and you are ready to go! Let’s get into the steps in greater detail.
Steps to fixing a cracked guitar top:
Step 1. Remove the strings and clean the guitar face. This will help with matching the finish after the crack has been set. There are many ways to clean your guitar, and the finish will dictate which product you use.
Step 2. Use the painters tape to tape as close to the crack on both sides. This just helps keep the face of the guitar clean.
Step 3. Apply glue to the crack. Use the little suction cup to work the glue into the crack. We want to make sure the entire length of the crack has been filled.
Step 4. Clamp it! A ratcheting bar clamp works perfect for this project. It can fit across the entire guitar and pull the crack together quite nicely. If your crack is longer, you might need 2 clamps. When you clamp, it will squeeze out excess glue. Make sure you clean this with a wet rag.
Step 5. Make a wood cleat out of a shim. I cut the thick end of a shim so I had a square about 1.5”x1.5”. This cleat needs to be glued to the backside of the crack on the inside of the guitar with the grain of the cleat being perpendicular to the grain of the guitar. This will ensure that your guitar will never crack in the same place again. This can be a little tricky. It may help to use a small mirror to help line you up. I used the remaining portion of my shim to make a little leverage stick that I could slide into place to hold the cleat up on the crack. This was very much a trial and error process for me getting the length correct. I rounded one of the corners on the shim which makes it easier to pull into position. It took me several tries to get the correct height right on the cleat.
Step 6. Sand down the repair area and apply the finish per instruction on the bottle. I had to repeat this step a few times to get a good match on the guitar. Again, this was much easier because I was working with a natural matte finish guitar. Matching paint or laquer is more difficult.
Once the cleat has dried in place and the finish has dried your guitar will be structurally sound and ready to string back up into action!
