To repair rotting exterior wood, first determine whether the rot is on a major structural or on a nonstructural item. If you have rot on a nonstructural item, such as trim, you can fix the rotted area with epoxy-based wood filler. Otherwise, replace the item.
Begin by locating the rot, giving special attention to areas that do not drain well as well as areas in which plants or other items could block circulation. Pay special attention to dark areas, and check wood that is part of a joint, is close to concrete or is horizontal. Pushing a screw into the wood is relatively easy on a rotten area.
Remove the rotted wood using a heavy tool, such as the claw on the hammer, a flat-head screwdriver or a chisel. If it becomes difficult to remove, use a power tool such as a die grinder. Remove all wet materials.
Use wood preservative to protect the undamaged wood, which is exposed, by drilling some holes into the wood and injecting the preservative.
Mix epoxy wood filler with its catalyst to make it chemically active. This limits the time for working with the epoxy before it starts to harden. Use a putty knife to shape the epoxy to match the rest of the wood, avoiding body contact with uncured epoxy. After the epoxy has cured for a day, sand it flush with the rest of the wood and protect it with a few coats of exterior paint.