Thanks to everyone for your comments.  A new old guy needs all the help he can get.  Tom

ISS are you saying the copper brush scratched the stainless steel in your gunsmith’s demo ?  I’ve heard this stuff for years and ran it by my wife who was a metalographer in an aerospace lab. She tried it using bronze and copper brushes on three different SS 300 series alloys then examined with her SEM ( scanning electron microscope. It’s not possible not matter what anybody tells you. Rockwell hardness of cold rolled copper is‘10’, yellow or cartridge brass ‘40’ while 304 SS is ‘88’ !. This is major. What I think you saw were copper bristle trailings or metal smears on the SS. 

Rick