Mike, most of those are done with a powder coat process. Metal sculpture which has been rusted won't usually hold a clear coat long term, unless it's done professionally.
I do some metal sculpture works where I want a very even rusted finish, and to get this I mist them with a solution of bleach and water, then place them in the sun to dry. They don't have to remain wet for this to work, they just need that initial damp surface, and the bleach does the rest. About 3 to 5 days later you have a fine, even rust finish.
I have done a few where I mimicked a powder coat finish by using black and copper spray paints simultaneously, spraying them randomly AND without shaking the cans first. This gives you the "spatter" effect you see in the rust colored powder coat finishes.
I'd suggest you play with scraps first, as the finish will vary with the grade and condition of the metals you use.