I would suggest trying "Barkeepers Friend". It comes in a container that looks a lot like Comet, but is gold. I find it in the grocery near similar kitchen cleaning products. It's intended to clean the bottoms of pots, but I've found that if you form a paste with it (water activates it somehow) and let that sit on a stain it will remove rust and many other stains. The fact that it is affordable is a nice plus. Good luck.