There is really no type of standard paint that will stop rust when it is constantly exposed to some type of moisture. Sure, a lot of people try and Kent has seen many failed attempts. The worst are trying to cover over it will some type of seal, sealer or body undercoating. That makes it even worse because those types of products will allow moisture back in and then "trap it" - thus making the problem worse by creating accelerated rust through!
Problem & Solution
Common among these chassis:
You must use a coating that completely seals the rust (encapsulates it) from moisture and oxygen. The best product for this is Miracle Paint. Yes, I know it sounds like a gimmick but it really does work as Kent has proven on rust repairs he has done on cars over the past 15 years. As you see here Kent was able to isolate and stop some beginning rust in his 1986 300SDL.