You can get one of those rust stripping things for your drill, they are some kind of hard plastic and they work pretty well. They work great for light rust and paint removal but you'll need to fill where you use it because they will gouge right through the paint.

http://www.vwtrendsweb.com/tech/0401vwt_super15_s.jpg

If you're grinding though, that will gouge through the paint and into the metal, so you might start with one of those rust removal discs on a drill before you go to the grinder.