Get some of the green 3M Scotchbrite scrubbing pads from your local grocery or dollar store. Cut off a piece about 1" X 1 1/2" - depends on die size. Slit the end of a short piece of appropriately-sized wood dowel, or wrap it on a brass brush, and use your cordless VSR drill to spin it. With a bit of patience and some in&out motion, you can put your die back to factory new finish. You may have to go small, if the scratch is in the neck of the die. I have done this a lot, and have never had it alter the size any detectable amount. It will even work on carbide dies; just takes longer.