How much rust are you talking about? I've cleaned up light patches before by sanding and applying a couple of coats of Rustoleum. If there's significant build up, you may have to bump it up to a rust converter, and I'm not sure if there are any OTC. The ones I've seen have all been online.
I'd order stuff from some place like EASTWOOD and do it right. It's not all that hard.
At the least- clean everything as best you can, apply their Rust Encapsulator (I'd use black) and then spray their Black rustproofing stuff over top of it. Won't be much more of a job than Rustoleum but it'll hold up infinitely better.
I just got my order in from RustBullit.com. I'm going to try it out this weekend and see how it does. I've used POR-15 and I didn't like all the steps needed to use it. This one is simple and I hope works out well
I have heard nothing but good stuff about this and also similar type products, but especially this one. When I get the time I am doing my whole under carriage in this stuff, but don't expect it to be pretty just functional. Look on the site I think they have examples of it on vehicles.
Their main compitition is POR 85, here's cpmarison.