I used apple vinegar once that worked, I put a lot in, added as much water then brought it to a rolling boil, turned it off and let it cool. Came upon the process by accident, I had been boiling apples to grind down for apple sauce, after I was cleaning up for the day I saw the large pot I used to boil the apples was cleaner than when I started. It was a very old pot with lots of stains, after that day it looked almost brand new.