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