I agree with all the "preparation is the key" advice. You really need
to work at getting to clean metal.

When I've done this I've gone with top notch oil-based primer and
finish coat, in quarts/gallons. If others have used spray cans and
gotten good results, I won't argue.

And I'd guard against rolling on really heavy coats of paint. If you
want thick paint it's best, I think, to do it in layers, and you want
the wrought iron to still look like wrought iron.