Oldfart
FRF Addict
I've had great luck with Rustoleum Rust Reformer spray paint to get your truck looking good again underneath. It comes in black, just clean things up as well as you can, then spray it black. I hand sanded off the rust I could get to. I did my axles and some other small pieces underneath that looked crappy as soon as I got my truck last year. I spent about an hour prepping and cleaning it up and then sprayed it. It still looks really good. It's SO easy, because it's already black.