I bought some Fluid Film and a 24" tube with a 360 degree spray nozzle and sprayed it inside the frame rails. Next fall I'll probably not only do that, but will hit most of the underside pretty good. The main thing is to just keep it clean and don't let rust get a beach head. My 11 year old JK had no rustproofing but looks almost new underneath because I'm a fanatic about washing the underside after it gets covered in road salt. Anyway, the Fluid Film is just insurance.