Epoxy primer will get a better bite on the metal than the urethane. I start with sanding with 80 grit and applying two full wet coats of epoxy. Within the recoat window, I then apply two coats of urethane surfacer and start block sanding from there. I try to have my metalwork really close before applying the epoxy primer. Epoxy primer gives more corrosion protection than urethane prime and it eliminates the old timey use of etching primer for bare metal.