I would check out Insl-x - they are owned by Ben Moore but sell some really good commercial/industrial coatings. I think for your application you could probably use their rust scat line. This is a very durable oil based enamel. Can be applied directly to metal but I would probably still prime the bare chasis. There are certainly some really good epoxies you could use as well, but they can be tricky to work with. There is one called Insl-tron which you might want to look up.