Repairing an exterior door casing involves using a pry bar to remove damaged sections of the old casing, cutting the new casing to size and nailing the new casing to the door jamb. Finish the job by applying wood preservative to the new casing and then priming or painting it to prevent water absorption.
To repair damaged door casing, first use a utility knife to scrape paint and caulk away from the casing. Next, use a pry bar to pull the casing away from the door. Use a chisel to scrape away any remaining traces of paint or caulk from the siding or door jamb.
Measure the height of the door, and cut the new casing to size. The cut at the top of each vertical length of casing should have a 45 degree angle. Cut the horizontal part of the casing to fit along the top of the door, again including a 45 degree cut on each size.
Position the freshly cut casing on the door frame, and nail it in place. Apply caulk to the nail holes, as well as to the edges of the casing, in order to seal them and keep out water. Apply primer and paint to the new door casing to further protect it from the weather.