You could try POR-15 for a primer. We find it is very durable. A local fertilizer company uses it to prime fertilizer spreaders. In my opinion it is better rolled or brushed. Pay particular attention to the handling warnings though because once it hardens it has to be mechanically removed. You even have to put plastic wrap (2 ply) under the lid after you open it or you can not get the lid off the second time(voice of experience). It is not UV resistant so it must be covered with some other paint to block the UV.
Meltedmetal