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I can always tell when we’ve had a cold winter by the guitars that come into my shop. When folks crank up the heat during cold weather, it removes moisture from the air. A guitar’s fretboard can actually shrink in a low-humidity environment, causing the fret ends to protrude slightly on either side of the neck. You can generally prevent this by using a room humidifier or inexpensive guitar-humidifying device, but once the sharp fret ends are poking out, you need to deal with them, because they make the guitar very uncomfortable to play. It’s not difficult to repair sharp frets, but it takes the right tools and materials, and you have to know what you’re doing.
Photo 1 shows an example of sharp frets. This is a fairly new guitar, but the owner has obviously never humidified it. At this point, the fret ends need to be filed, shaped, and buffed.
You need two files for this job: a fine-tooth flat file (Photo 2) and a three-corner miniature file (Photo 3) that has been customized for fretwork.
Fine-tooth flat files come in many sizes and shapes, and they’re available from specialized guitar-tool companies like Stewart-MacDonald, Luthiers Mercantile, and Allparts, and even home improvement stores. Look for a file with fine teeth that’s between 3 1/2" and 8" long. Several companies carry diamond-coated files, and these also work well.
I prefer Nicholson three-corner miniature files, but most brands get the job done. A three-corner file is perfect for rounding the sides of a fret end, and the best type for this job is a “single-cut” file. In other words, the file’s teeth only cut in one direction, which limits debris buildup.
Important: Make sure you round and sand smooth one of the corners of the file. Otherwise, the file can cut into the fretboard (and binding, if the guitar has it). I used a belt sander to gently remove the teeth from one of the edges of my file. If you don’t want to do this yourself, the guitar-tool suppliers mentioned above sell three-corner files with smoothed leading edges intended for fretwork.