You really should use a non-conductive abrasive. There are special commutator cleaning stones available, lacking one of those, old fashioned flint sandpaper (extra fine) also works.
An X-Acto razor saw blade (without the handle) is ideal for under cutting the mica insulation between the commutator segments.
For general cleaning, I've had pretty good luck with the cheapest brake & parts cleaning aerosol cans I can find, usually a case of them from BJ's.
Roy
