Protecting a finished wood exterior door
You mention the door is "finished" by the manufacturer. Can you tell me what type of finish ? Most exterior doors that are stained can be sealed with a product such as spar varnish, or marine grade epoxy. Seikens makes some very good marine sealers that work great on exterior wood doors exposed to direct sunlight. If your door is shaded and protected by a storm door, than  it can be sealed with urethane, however urethane is not recommended for direct exposure to the elements or direct sunlight.