I'm new to reloading. I had some OLD 30-30 brass that needed cleaning (IMHO) prior to reuse. Several had green "mold" that scraped-off, but left an awful brown stain on the brass.
I tumbled the brass in rice...worked great but the dark brown spots still remained. I tried several remedies including 0000 steel wool...tough stains. Then I tried "Bar Keepers Friend" that my wife keeps under the kitchen cabinet. BAM! Stains gone with minimal scrubbing with a paper towel! It's the same consistency as comet or Ajax powder.
Y'all probably already know this, but it was new to me.
Good hunting.