I bought this cookware set about 3 months ago. It is a good quality product. My only concern is that the bottom of the pans are starting to get discolored. I cook with a gas stove, so not sure if that has something to do with it or not. Is there something I can use to get rid of the brown stains? I'm thinking Comet or something along that line but didn't want to use something harmful to the pan.