Definitely look into using an adhesion promoter such as "Bulldog". Spray on 2 coats of this stuff on sanded bare metal before your primer. Make sure you use some degreaser such as prep-all to remove wax & oils. Everyone who suggested powdercoat is right, it is the toughest coating out there but it may be beyond your capabilities/equipment/budget. BTW this is my first post but i have been reading the forum for a while, Really great stuff here.
