The best thing to use to clean black marks left by pots and pans, etc. on your cast iron sink is a Magic Eraser by Mr. Clean. There are also "copycats" out there that work fine. This wonderful cleaning tool is also great for stainless steel sinks. I have had white cast iron sinks for over 47 years and I can tell you from a plumber's wife's experience you should never ever use an abrasive cleaner or a pad that will scratch the surface on cast iron or stainless steel. The Magic Eraser pad is truly magical on a myriad of surfaces. Try it. You will love it.