Scotchbrite 3M Rolloc discs on a drill or die grinder. Just take the burned off. I use a 1/4 sanke to mash in. If I do a step mash, I usually (always) burn the bottom a little. I clean it up with the roloc disc on a drill. I had my then 7 year old son do a demonstration at a club meeting. The steel will passivate its self. I don't worry about it.
David