Browsing the forum came across this thread. I use a product called Rust Check (coat and protect) in a 350 gram rattle can. This product goes on kind of gooey, bubbles around the surface and then just "hangs" there like a gel. After a few months, I look at it and it still looks gooey, like I just applied it. I don't do the whole underside, just what I think are critical areas that might rust over time. It is made in Canada by "Rust Check Corp" in Mississauga Ontario. I use it to spot spray exposed steel areas, and fasteners. I also spray the fuel and brake lines especially where they are mounted in steel or rubber clips. I put a can or two on my Christmas list for the missus, that way I get it for FREE! I used to use a product called Jig A Loo, also one of my favorites. I have no affiliation with either of these companies.