Have you considered using corrosion prevention compounds instead of paint?
I am currently doing a test on several Corrosion Prevention Compounds at home. I started with 6 or 7 products that I applied to bare metal plates. It's been going for a few months - I spray all samples with salt water spray about once a week, after the first month only 3 products held-up well.
Based on that test I can recomend Eureka Fluid Film and CRC Heavy Duty Corrosion Inhibitor. They come in aerosol sprays and don't require any surface preparation. As long as you don't mind the appearance, they will stop rust for at least 6 months. I've been using them on underside of my cars and boat trailer.