If acetone, MEK, citrus solvent, toluene, gasoline etc. don't clean them up, try vegetable oil. Its a pretty powerful solvent, sometimes works when other stuff doesn't. Then try Scotchbrite, the red ones (fine) with some WD40. It takes quite a lot of work to remove 0.0001 with scotchbrite, so you will have to work really hard to damage them, but its pretty good at getting paint, oil varnish, etc. off. The WD40 keeps it from clogging. Wet/dry sandpaper, even in fine grits (like 600 or 1000) will remove more metal quicker.