I love this topic. I'm so meticulous about my undercarriage that I worry about my mental well being, lol. If I see rust I eliminate it ASAP. Over the years however I've devised a system that has worked better than just about anything I've seen. I just touched up my frame this spring for the first time in years, and it was mainly just because the coatings were starting get white and chalky from exposure, and a few areas were peeling up. Everything underneath was clean metal, however. Anyways the first thing I do is a typical frame cleaning which concludes scraping, sanding, wire wheeling and a power wash/wipe down. This will get it cleaned up nicely, especially if it's never been done before. Then I use a rust converter product, like the blue can spray stuff from Permatex or the like. After a liberal spray I let it dry then use a spray primer from Rust_Oleum (rusty metal primer typically). A general coating of black spray paint (semi-gloss or whatever is on hand)gives it a nice base, then a top coat of DIY bedliner. If I'm feeling really generous, I shoot a layer of rubberized undercoating in the wheels wells for stone chip resistance. It can be a lengthy process, but It's so durable afterwards. A long hard winter up here in Ohio won't even touch it.