I would like to start a on frame driver restoration of a 1960 Triumph and would like to ask how to go about in starting this task. Some areas of the underbody and frame have minor surface rust where paint has chipped/worn. My understanding is to leave areas alone where the paint on the frame and underbody remain in good shape. My plan is to treat the rusted areas with phosphoric acid but I am not sure where to go from there in coating the spots as well as painting/undercoating the remaining frame and underbody. Do I need to scuff the old frame enamel (Triumph’s were originally painted with tractor enamel), coat it with primer (which kind, epoxy or etch), paint with chassis black and then apply undercoat or use undercoat alone?