Exterior doors take a beating from season to season. Constant exposure to the elements really wears on the finish. At some point in the life of the door, it will be necessary to strip off the old finish or paint and apply new. This keeps the door looking great and preserves the wood for many years to come. This is especially true if it's an old door. After stripping away the old paint or finish, the key to a great-looking exterior door lies in the preparation of the wood before refinishing. Sanding is the key to that preparation.
Place the exterior door on blanket-covered sawhorses. This keeps the sawhorses from damaging the bare wood of the door. Remove all hardware, such as hinges, door knobs and back plates.
Sand the door with a random orbital sander equipped with an 80-grit sanding disk. This helps remove any old paint or finish that the stripper might have left behind. Sand all of the flat surfaces of the door. Once you have sanded the large flat areas, sand the inside and outside edges of the door if they are getting refinished.
Change the disk on the sander to the 120 grit. This will smooth the marks left from the more aggressive 80-grit disk. Again, sand all of the large flat surfaces of the door. If the door has recessed panels, sand as close to the edge of the panel as you can. The rest you will sand by hand.
Switch to a sanding block wrapped in 150-grit sandpaper. You will sand the rest of the way by hand to remove the swirl marks left from the orbital sander. It's important to first sand the horizontal parts of the door, known as rails. That way the scratches left on the longer vertical parts, called stiles, will be sanded away.
Sand any raised or inset panels with the sanding block and 150-grit paper. If there are any moldings, fold a small piece of sandpaper and use the crease to get into hard-to-reach areas.