I had decent luck dealing with similar issues on my son's old Protege5 using POR15 to arrest the rust and provide a base coat. It bought us about 3 more years before the rust returned living in salt-happy Michigan. Had we re-coated, we might have even got more life out of it. You actually don't want to make the surface too clean with POR15, it needs something to attach itself to, so prep is pretty easy. Just remove everything that's loose, coat it, let it dry, top coat it with paint. Lots of decent "how to's" on the tube of you's.
Just a word of warning: If you get it on your skin, you're going to wear it for about a month.