I've chucked the valve stem up and spun them up in a drill press or lathe before to pretty just the face up for the customer to see in the chamber on head jobs, but my point is that anything abrasive enough to clean the back of coloration is risking the sealing surface, and that's unnecessary risk to me, as clean is functionally perfect and you can't see it once assembled anyway. Cleaning the face can be any clean-cutting abrasive including Scotchbrites and sandpapers. Something like lapping compound on a rag or leather pad would leave it silver but dull.
David