I've had good results with metal files. I use a chainsaw file (round, 1/8"-3/16" dia) for monochords and bi-chords, and a flat file for the flats. For flats that have dried gook on them from rusted strings, dried liquid, etc. I have used a small triangle file (maybe 3/16" per side) right on the string grooves. I can file them with the dampers in the piano. If the dampers are too far gone, doing this will yield moderate results but they should probably just be replaced. -Matt
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