Try wetting the surface, applying Zud and rubbing with a nylon scrub pad.
Navel jelley will do it. Or go to a boat place and ask them for a stain remover. I can't remember the name it is blue in color that will work also.My daughter had a metal teapot in the bath with her. It left a rust ring on my cast iron tub. How can I remove it without damaging the finish?
Tub is 12 years old. Rust has been there for a while. Have not tried any abrasives to remove it.