I would scrape all the loose and peeling paint, then hit it with a wire brush, then scrape some more (if it is peeling really badly) then sand with sandpaper trying to remove as much rust as you can.
you will have to prime with a rust inhibiting primer. I dont think the heat is as much of a problem as the type of metal you are painting - is it steel or galvanized? if it is galvanized you have to use acrylic paints or it will most definitely peel more quickly.
if it is steel you should probably get a spray can of rust inhibiting primer - it will cover the perforated metal better and you will not have to clean a brush with thinners.
I would go with an acrylic top coat. with the new VOC regulations that will take affect nationally in 2011 why put a material on that will not be available in a few years.