Rust never sleeps
Rust never sleeps in south Texas due to the salt air environment but you can slow it down.
After a trip to the beach, Port Aransas, Padri, what ever I hose off the underside of my truck then after it drys I'll take a pump up sprayer full of a75/25 mixture of diesel and motor oil and spray the undercarrage real good. Spray the whole thing and get in all the nooks and crannies.
In the old days they used fish oil to undercoat cars and trucks around here but I havent heard of that being used since the early 1970s.