For my metal sculptures, I almost always use AwlGrip marine urethane.  This is the paint that is used on America’s cup boats, Fed Ex jets and a number of architectural items.  While it is some of the most expensive paint available, the value is in the longevity of the reds and yellows.  Nobody likes to repaint anything, but it is a reality for anything outside – houses, cars, yachts – they all get repainted sometime.