Sand off as much rust as you can, then prime with Pettit Rust-Lok. It's a 1 part brush on primer that can be brushed over light rust and if you do 2 coats will seal out moisture almost as well as an epoxy paint. The stuff sticks to anything, especially your hands, has decent solvent resistance, isn't very expensive ($30 a quart but gets very good coverage). You can get it at most boating equiptment stores. When you're ready for paint, sand it smooth and do your normal painting regiment on top of whats left.