Some time back, I bought an OLD aluminum colander that I got from a garage sale for a dollar. Being from a garage sale, I wanted to make sure it was good and clean before I used it...right! I had it sitting in hot water in the sink with dish liquid soap (like Dawn) and after I took it out, it was an ugly gray color. Just sitting in the dish soap made it discolor. I have tried everything I found on the web and then some to try and clean off the blackened surface. Nothing touched it. Finally I drug out the BKF and wouldn't you know it, the black has polished right off! (I used a sponge) That stuff is amazing. I was just fixing to go and buy some Weiman's cook top cleaner that I had read about is supposed to take it off too. I suspect they are similar. I used the powder version of BKF btw.