By prep if you mean primer, etc. then you can use an epoxy primer. Or if the original finish is in good shape, you can sand it and paint over the original finish. But any rust or bare metal spots should be coated with primer first.
Whatever alkyd enamel you use, you should get the tech sheet from the manufacturer and follow that.
I've used PPG's ALK-200 and it's pretty easy to use. You can either spray it straight from the can, or add ALK-201 hardener, and thin with Xylene if needed.
