Never dull is used for military buttons, etc.
Here's a video tutorial: https://www.youtube.com/watch?v=r-cSrK_o25A
I found this online:
Take a bowl and measure in one tablespoon of flour, one of salt, and one of vinegar. Mix it thoroughly until it turns to a paste, and then rub it on to the surface of the brass or copper with microfiber. Leave it for a couple of minutes, and then wash it off with hot soapy water. Then rinse it and buff it with a soft cloth. Your brass or copper handles are restored!