An epoxy primer would work well. PPG makes a primer called DPLF90 you could spray a good primer base with it then for the second coat thin it and it acts as a sealer. It has a very even satin finish to it. As delux54panel stated primer is very porous and will let water pass right through it and you will end up with rust under your primer so you would want to finish it with a sealer or paint.
Godd luck with it.
