Originally Posted by JeffTsai

Get some sandpaper and sand/clean off the rust so its just down to the bare metal. If you don't want to use sandpaper, there are also those liquid rust removers. I've never used the liquid ones so I don't know how well they work. Anyways, after you get rid of the rust clean it off so theres no dust or oil there and put some touch up paint in that area. If it's under the car then just shoot it with some of that liquid underbody sealant stuff. I think Lexus coats the whole underbody with a ton of that stuff to keep the road noise out.