Pull the parts out of the gun. Get a small piece of plate glass and some 1200grit wet or dry paper. Put the paper on the glass and wet the paper with light oil (RemOil, 3 in 1, WD40, Ballistol, etc.) and rub the parts to the finish you want. You could do it with a Dremel but you stand a chance of taking off too much metal, rounding off any sharp edges...or worse.