I just finished doing the entire underside of the vx before Moab. I had the front bumper off for some other things and got in there to take care of a little rust. After steel brush and a rust removing wheel on the grinder I shot it with this "Krylon Rust Converter". It chemically stops the rust and dries almost like a plastic coat over the top. Then painted over it with some rustoleum. When you spray this stuff on rust it turns the rust black.

Rustoleum makes a similar product that I read the entire label on. I dont remember why I ended up with the Krylon but I am very happy with the results.