For the combination of price + good results + availability, MINWAX products are a good choice. For exterior woodwork, I like their Helmsman Spar Urethane. First thing is remove the old coating (shellac, lacquer or varnish). After you have the door cleaned-up and sanded (the grain was raised during the stripping process) apply a Tung Oil finish to add additional oils to the door surface before applying the urethane.
 

  1. Let the Tung Oil cure for 30 days.
  2. Lightly scuff sand the door with 220-grit sandpaper.
  3. Remove sanding dust using a soft, lint-free cloth dampened with pure mineral spirits.
  4. Let mineral spirits evaporate (door dry off)
  5. Apply 1-3 thin coats of urethane following the label instructions