Original factory paint is an acrylic modified enamel. Any thing will go over it if it is sound and properly degreased and sanded. If it has acrylic lacquer over the factory paint this can also be over coated with anything as long as prepared as prev and is itself sound with proper adhesion to the factory paint. Sand and treat light rust with phosphoric acid then rinse with a damp cloth as per instructions. It must be non sticky when dry. If any unreacted acid remains it will inhibit the reaction of the epoxy and it will not cure. Such means full removal of epoxy and reprep. Basically epoxy is a good choice as it has good adhesion and as long as a good build ( about 60 micron min )is totally waterproof. ie 2 or 3 sprayed coats or same by brush. Once cured and sanded with 180 or coarser to give a key it can be bogged over. This is better than bogging over straight metal unless the metal is new and sanded to provide a key. The epoxy gets full hardness in a week so best to have another fresh coat when applying colour of any type. ie prime again then topcoat within a couple of days.