Cleaning and scraping is a good idea....they do make a specific paint for that application. Benjamin Moore makes an exterior paint designed for metal (chain link fences especially.) It is weatherproof aluminum (colored) paint...their product number is 16478 and it is available in gallons and quarts however I'm not sure about the availability in your area. This product is designed to go on bare metal with no priming necessary. If that isn't available in your area they also make a self priming metal paint called ironclad. It is available in thousand of colors...the product series is 163 at your local Moores dealer or z163 if you need a voc compliant product. Both these are oil-based and can be rolled, brushed or sprayed (when thinned slightly.) If you afre going to spray yu might want to use a HVLP(high volume low pressure) sprayer that can be rented at home depot for a small fee. They work excellent and have minimal amounts of overspray. Good luck