I decided to look under the plastic top cover of my 2010 Rangers back bumper and it is a mess under there. All scabbed up and rusty. I work around boats and ships in a dockyard and I was wondering if there would be any setback to my scrubbing this bumper with a wire wheel and then priming with red lead oxide primer and then painting with marine enamel paint then replacing the plastic cover. This type of paint when paired with that primer lasts for years on the bottom of a steel ship. It should last the rest of the trucks life under that bumper. I also took the plastic bed liner out and it is similar on the inside of the box but not so dramatic as the bumper. I will likely do the same in there and then put the liner back in. Anything i should know before proceeding?

 

Marine paint is fine to use but it's main benefit(cost) is that it is non-toxic to marine life, an experience I hope your Ranger avoids, lol.

After cleaning use Naval Jelly as instructed, that will change any remaining rust to a neutral surface coating.
let dry and prime

Marine paint, as said, is fine but any exterior rated paint will be fine as well, hull paint often doesn't hold up well in direct sun light, some do, but not all.

Primer bonds to metal, paint doesn't
Primer can't hold up to moisture and sun
Paint can bond to primer, and can hold up against moisture and sun, exterior grade.

 

Is marine paint more expensive than regular automobile paint?

 

Can be, automobile paint can get pretty darn expensive.
But generally marine paint will be more expensive because of the requirements to be safe for marine life.