You will need to remove the door from the hinges, use a sander or chemicals to remove the old finish and then apply a new finish according to its directions.
Weather-rotten door can be given a new finishing touch by the simple touch of kerosene oil on the spoiled part followed by a good quality paint coating of two layers.
First you strip it with chemical or by sanding and scraping. Clean it up with some fine sandpaper to remove scratches. Then you just apply your finish.
