Jerry W.
This is actually very common but also somewhat tricky. You really have to make sure you clean the surface very well first things first. I would lightly pressure wash it to blow off any remaining flakes of paint. I would caution you against using any mild acidic cleaning agent, a strong degreaser (Krud Kutter) would do just as good, just make sure you don't use a super cheap one because it might leave an oily residue. You will then want to use an exterior, DTM (direct to metal) primer. This will somewhat "dig in" to the metal, you can use latex and you will probably end up paying $35+ per gallon. You have the option of grey, red oxide, or white and it really doesn't matter which you decide to use. If you can find a "high build" that will be even better because it will help bridge the gap where the paint flaked off. If you use a high quality DTM primer you can then use either a regular exterior paint or a DTM paint finish for your topcoat. You will want to use latex base, it will hold up much better with the metal believe it or not, after one good summer of baking the paint on it will last for many years.