I use used motor oil. I save the oil from all my oil changes and use a undercoating gun with a air compressor. I also take the tail lights out and spray up inside the bed from the tail light hole. Works like a champ!

 

My right front fender rusted out were I drilled the hole for the mud flap, rust was starting on the out side fender near the drivers door, I was able to save the fender, and patch the hole with metal (roof flashing) now I need to work on my inner door seams as thier rusting again.

CLR helps take away some surface rust

 

I bought an 03 ram and there is no rust. I'm trying to prevent it as much as possible. I bought some texaco compound L. The design flaw that causes the rust are the holes under the wheel well that trap salt, etc right? So I would need to get the compound L inside the bed of the truck? What would be the best way of doing this? Thru the taillight area?