I've had good luck with tremclad (rustoleum for you folks south of the 49), When cured with a little heat. Like on the rad tank, and block. It takes a few hours of running, but the paint gets much harder, particularly, if you use light coats, like the darn instructions say.
Easy to touch up is the best! It's amazing just how fast paint, and spray on undercoat wears under the truck, and in the fenders particularly. I drive in allot of rain though. A little paint goes a long way...
http://i221.photobucket.com/albums/dd45/HomeCrafter/IMGP1761.jpghttp://i221.photobucket.com/albums/dd45/HomeCrafter/IMGP1770.jpg
