I have never used Summit's paint,but I do paint!The epoxy would be first to bare metal.Urethane primer would most likely be a build primer to cover bodywork and surface imperfections.I use House of Kolor or PPG paints at my motorcycle shop.The topcoat dictates the primer,I use the same manufacturer from primer to clear coat.The HOK primer is Epoxy that adheres to bare metal and fills scratches,they do not have urethane primer.PPG uses a wash primer or epoxy to bare metal,then urethane primer for a filler.
I'm not sure who makes Summit's paints,might want to ask them.Probably want to use the entire system if it seems ok.
RT
See some of my paint:
www.northhillscycle.com