[nsb29]

The best thing is to keep it simple strip it to bare metal, wipe it down with a wax and grease remover, give it three coats of SPI epoxy primer, if there is no welding or body work to be done apply your high build primer within the recoat window.
If it goes beyond the recoat window it will need to be scuffed/sanded and degreased before applying your high build primer I prefer polyester high builds like slick sand or feather fill over the 2k primers