If they are just steel, then either way you paint them (rattle can or spray gun), prep is going to be your best friend. I would suggest you clean them very well and wash with a soap made to remove grease and oil.
I have always had the best luck if I rattled can something to use a product that will actually etch the metal before applying the primer. Ecthing the metal will allow the primer to adhere better, since you are not using a hardener like with a air spray gun type. You can find all kinds of products sold at auto/paint places in rattle cans that will etch the metal.