I painted my two horse trailer with a product called Hamerite.
All you need to do is clean all the grime off and scrape any loose
paint off. My trailer was orange and I painted it blue. HAMERITE
eats through rust and old paint. No primer needed. Over eight years
the color faded a bit, I am really pleased with its durability and ease
of use. I did use a paint gun, and you have to put 4 coats on with
(I think} in a hour per coat.