Quote Originally Posted by rondog View Post
OSPHO kills rust quite well, so I've read on Jeep forums. Chemically converts it from iron oxide to iron phosphate, or something like that. Basically phosphoric acid I believe, but do your own research.
It does "kill" it but it is pricey, $50.00 a gallon. And it is not something to indiscriminately spray on the underside of a vehicle, it will fuck with wiring etc. And it needs to be neutralized as well.
Power wash underside thoroughly and allow to dry. Spray with either used motor oil or even better, linseed oil.

Things like undercoating, POR15 etc merely cover rust, the metal continues to rust under the coating. They claim they will halt rust but it wont.
Rust is like fire, it needs 3 things to grow - oxygen-iron-moisture. Remove one of those things and it will halt growth. The oil seals it against oxygen and moisture.
Reapply annually.