Exterior wood doors are subject to the elements, so it is wise to check them for dry rot and pest damage every so often. Even hardwoods, such as redwood and cedar, are prone to damage if left unsealed. Catching the damage before it has progressed to a point of no return will save you time, work, and the expense of a brand new door. With a toolbox full of common tools and supplies, even the uninitiated homeowner can successfully repair rotted wood in an exterior door.
Insert a screwdriver into the rotted area of the door. If it penetrates more than 1 inch, a new door is in order. If the screwdriver hits sound wood at less than 1 inch, the door can be repaired.
Remove the door from the frame and lay it on a firm and level work surface. Wear goggles, and using a hammer and chisel, chip away all the damaged wood from the door. If the rotted area is wet, allow it to dry. If the rot is near an edge of the door, tack a straight piece of wood to the edge to create a form which will make a boundary for the filler. Be sure to coat the wood with vegetable oil first so it does not stick to the filler later.
Drill a few shallow 1/8-inch holes, in a honeycomb pattern, into the damaged area.
Use a putty knife to mix and apply the two-part wood filler according to the product's instructions. Smooth, level and shape the filler to the proper form of the door. Cover the area with plastic and level the filler again with a small block of wood. Remove the plastic; allow the filler to dry thoroughly until it is hard.
Remove any forms if necessary. Sand the filler lightly without over-sanding. Wipe the area clean and prime. Let the primer coat dry for two days before painting the repaired area to match the rest of the door. Remount the door in its frame.
Save on expensive wood filler when repairing larger areas of rot by filling the hole first with shards of scrap wood. Generously apply wood filler to the cavity, press in the wood shards, then apply more wood filler over the whole area. Two-part wood filler is key. Not only does it easily mould into shape, but it accepts stain and paint well.