Sorry I missed this one last week. There is a product you can find in the clean & polish section of your local grocery store called "Rust Free". It comes in a hand spray bottle and works wonders to remove rust stains from auto paint. It is literally a spray on, set 30 seconds, spray off product. Repeat as necessary.
When I got my truck, it was covered on one side with accumulated rust stains from years of parking where a lawn sprinkler hit it. It took a few applications (three IIRC) to completely remove the rust stains, then a quick polish for good as new.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s