I use a Magic Eraser and Dawn dish soap on a day-to-day basis, both of which are magical when it comes to soap scum. When I first got our cast iron tub on Craigslist, I was able to clean up most of the rust stains with a rust stain remover from the hardware store. I think I also use a WET pumice stone on some of the tougher spots. Our finished is damaged in a couple of tiny spots, and there's no fixing that without having the whole thing professionally painted, which I don't want to do; we just have to call it a patina :)