Rust Stain Removal

I had this same problem. I left a light colored tank top on the radiator when it was wet and it got rust stains. You can try to do two things out. One, you can soak the shirt in water mixed with cream of tartar, which you can get at the supermarket. Or you can get a lemon or even lemon shaped bottle with lemon juice in it (I think it works best) and rub on the stain, then put it in the sun. I did this for a few days with the shirt, and it is gradually reduced. Do not worry if you have any spots with lemon juice, because after my spots were barely visible, I put the shirt in the wash with Shout! where the lemon juice and rust stains have been and shirt came out well, and there is no rust anywhere on it! This works because the lemon is acidic Oxyalic decaying rust. It makes a chemical reaction.

2nd what Daisy said! I have heard of the method CofTarter but I used lemon juice & Sun method several times and it works like a charm!

Wal Mart, Home Depot and Lowes all carry a product called RustOut and another called IronOut.
You may have to try both to see if your spot is really rust or if it is iron. Look for a white bottle with red label and blue.