REDGREEN,

I just got my '40 GMC cab back from blasting and it also has been epoxy primed. I turned the cab on its nose, scuffed the epoxy and applied an ample ammount of 3M paintable undercoat. When the undercoat dried I then re-primed with epoxy and finish coated with one-step black urethane with a flattener added so that the cab bottom would be the same satin finish as the frame. You should always scuff epoxy primer if it has been allowed to dry for a while. I am not sure if you need to re-prime the undercoat but I always use an epoxy primer or sealer before finishcoating as it prevents bleadthrough and is supposed to provide the best adhesion for finish coat. I added the undercoat as I plan to drive this truck regularly and this should help to deaden exterior noise. Hope this helps. gelee