Fret-crowning files have concave grinding surfaces that run along their edge. Typically each edge is a different size to accommodate different widths of fretwire (Photo 6).
Once you choose the appropriate edge of the file to fit your fretwire, place the file along the top of a fret, and glide it forward, filing in only one direction. Use slow, gentle strokes to remove metal from each side of the fret, while leaving the top untouched. The goal is to produce a nicely rounded playing surface with a peak that runs along the center of the fret.
After every few passes, clean your file with a wire brush to prevent metal build-up in its teeth. Repeat this process for every fret. Some frets will take longer than others to crown, so take your time and be thorough. When the job is finished, you should see a thin line of scuffmarks running along each fret that’s about the width of a high-E string.
When the frets are all recrowned, change the sandpaper on the leveling bar to 1500 grit. Using a light touch, glide the bar along the frets parallel with the neck to confirm that the fret tops are all level. If they are, then glide the leveling bar over the frets once more, this time working along the length of the frets, perpendicular to the neck. This will remove the scuffmarks and smooth out any inconsistencies on the crown of the frets.
Finishing touches. Now we enter the clean-up phase. To remove any tool marks left in the wood by your recrowning file, carefully scrape the fretboard with a razor blade. Exercise caution and don’t push too hard. If you slip, you might gouge the fretboard or even the frets
you just worked so hard to reshape.
With any tool marks removed, you’re ready to polish the frets with 0000-steel wool to make them look perfect.
Tip: Whenever you use steel wool around magnetic pickups, first cover them with painter’s tape to prevent little metal fibers from collecting on the pole pieces.
Finally, reinstall the string nut, put on a new set of strings, and enjoy the feel of freshly leveled, crowned, and polished frets.