Sand blasting is good! I still use etch, and lately epoxy sealer instead of the zink primers!
Auto paints dont stand up! An Industrial machine enamel paint used on laths and presses and mills, and industrial motors will! On sulfuric acid reactors The motors still look good after years of use with acid slashing on them every day! Zinc Chromate primer if you can obtain it !Zinc Chromate is a corrosion resistant agent that is added to certain coatings. Even today, chromate finishes including Zinc Chromate provide superior corrosion resistance. ( Zinc Chromate is highly toxic) it protects the surface from proliferation of organic matter, acids, salt,etc. Its in a lot of Machine finishes!
I take for grated this is a working trailer not a weekend horse show show piece! And your on a budget and want it to last a while but be able to take the abuse horses can dish out and working around your stables!