Rust stains on porcelain bath tubs or toilets:

The best thing to use on these situations would be a pumice stone. First you will need to clean the tub or toilet with soap and water, just to make sure there is no grime left on the area. Then use a pumice stone (soaked in hot water) and scrub the rusted area. Rinse thoroughly afterwards.

Rust stains on stainless steel:

Stainless steel should never rust, unless it has been left dirty or wet for too long. On this case the best thing to do is to scrub the area with a sponge and soap and try to keep it dry to prevent from happening again.