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 two things to get it 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 that bottle shaped like a lemon with lemon juice in it ( I think this works better) and rub it into the stain, and then put it into the sun. I did this for a few days with the shirt, and it gradually lightened. Don’t be concerned if you get stains from the lemon juice, because after my stains were barely noticable, I put the shirt in the wash with Shout! on where the lemon juice and rust stains were and the shirt came out fine, and there are no rust stains anywhere on it! This works because the lemon has Oxyalic acid which disintegrates the rust. It performs a chemical reaction.