I go to a place up near Concord that does oil undercoating, it was done by these folks by the PO for the last 6 years and I did it last year, so far I don't have any significant rust
If you have no rust use AMSoil HD metal protect. It goes on like fluidfilm but does not wash off like fluidfilm. Give it a couple coats. Then if you want...spray the cap out of it with fluidfilm. 5 years in my jeep using fluidfilm and 0 rust.