Exterior doors are not always an ideal candidate for any film-forming finish which includes epoxy. While slab exterior doors or exterior doors that are well protected from the sun and rain are candidates for an epoxy finish, most exterior frame and panel doors will fail quite quickly. The reason being that frame and panel doors are constructed using joints that allow for the natural movement of the wood to occur. When you close these joints with epoxy they can no longer move freely with the wood and in the end, the high strength, waterproof characteristics of the epoxy are no match for the stresses generated from wood movement induced by moisture cycling or thermal expansion and contraction. If the joint fails, a gap will open up that will allow moisture behind the epoxy, at this point it is only a matter of time before the epoxy will fail.
Under conditions where film-forming finishes are sub ideal using penetrating oil like Sikkens is usually the best course of action.