I've got a steel boat. The paint below the water line is two-part epoxy. I'll tell you this much, the most expensive paint I can buy for above the water fades to the point I cannot spot paint in less than a year. I wouldn't use it for a vehicle is just doesn't seem of good enough quality. I suspect that manufacturers know that steel boats have to be repainted every two or three years and don't concern themselves with UV protection.
