I painted my rocker panels, and the lower half of my doors with plasti-kote, which is just a rattle-can version of a bed-liner spray. It worked out great, and definitely reccomend it. Just be sure to sand down any areas with rust, if you can't get all of the rust removed, hit it with a rust-converter. Then you just need to scuff up the paint so the liner has something to adhere to. Here's a pic of my tracker, crappy picture, but you can see how it would look.
(http://smarttimmy.com/cms/images/stories/Tracker/phone210.jpg)