Nothing you can do to them in terms of coatings is going to be durable. Paint, bedliner, etc, is going to scratch. Your best bet would most likely be using gun black. You can get it at WallyWorld.
I'd remove the metal and give it a good cleaning with an acid, then color it.
"To do is to be" - Nietzche
"To be is to do" - Kant
"Do Be Do Be Do" - Scooby Doo