On my Last Trailer project I went to the nearby TSC (Tractor Supply Company) and bought a gallon of Tractor/Implement Paint, this stuff is tough!!! We used this on the frame of our old race and this stuff takes a licking and keeps on ticking!!
I would recommend this stuff to anyone as long as you can fine a suitable color. It is oil based and is very smooth when it flows out. I applied it using a brush.