Get some very fine sandpaper. Try to get most of the flashing off with a hobby knife, and then gently sand the bottom.
After sanding, I give each key a bath in gentle cleaner to remove traces of mold release and resin dust, and scrub them gently with a soft toothbrush. I then dry them off and inspect for any frosting.
Do be aware that you need a mask if you're going to sand keycaps. Breathing in resin dust is quite toxic and nasty, and you will breathe it in without realizing it.
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