Because you want "affordable" you should still get something reasonably decent so look for an Alkyd enamel like Rustoleum. Using a gray alkyd primer first will add much better corrosion resistance and color hold. A DTM (direct to metal) paint is no substitute for using a primer. Primer is primer...paint is paint... you can't mix the two in one product and have an equally good finish.
Stay away from Pour 15 and similar products because they are expensive and don't do as advertised in my experience.
