Good looking mahogany door. The first thing you need to find out is white type of finish is on the door, likely a form of varnish. Take a cotton swab and in a discreet area apply a small amount of acetone. If it is varnish the finish will become tacky. If it beads up it is polyurethane. Shellac and lacquer are not used on exterior doors. Next you have to decide how much work you want to put into it. To completely refinish the door I would recommend removing it and having it professionally stripped. Retail strippers sold to consumers do to EPA will not get the job done well. If you just want to clean and refinish I would wash the door thoroughly with TSP, rinse and let dry well. Once dry, and you have determined the finish type, scuff sand the existing finish enough to create good bonding and refinish with the existing finish. If you have the door stripped you will want to wash it well with mineral spirits and give it a day or two to flash off completely. Sand with 2200. If I were going with a new finish I would use Sikkens Cetol or similar. Do not use a polyurethane, the finish is too brittle for exterior doors. Cetol has excellent UV blockers to keep your door looking good for years to come.
