Aside from a good belt sander I've had good success with Gorgomyte cloth (but don't used it like the directions and rub the fret dirt into the wood).Ahead of our first show on July 2, I was just lemon oiling the fretboard and polishing the rest and wow do my frets look like crap. They feel smooth, though. I guess they just need a polish?
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Gorgomyte Fret And Fingerboard Conditioner
GHS has a version as well.
This works well too:
MusicNomad FRINE Fret Polishing Kit