How do i protect my vehicles frame from rusting from road salt and other things?

right now my jeep wranglers frame is rusting away, so im currently trying to find a new vehicles to fix that one, but im concerned on how to keep that one protected so i dont go threw this again, my dad said to just put undercoating on and the rust stops, but i thought once rust starts it keeps going unless you remove it, is there some sort of spray or cleaner i use to stop rust, rather then just cutting and welding new pieces on, any tips on what i should do, i live in pa, so we get a fairly salty winter lol, i just dont want to end up with two rusting frames