Hollow core doors are commonly used in residential construction. Although they are not made of solid wood like exterior doors, hollow core units are not truly hollow either. In most cases, they are filled with resin-coated cardboard or foam surrounded by a thin layer of wood veneer. The resin-coated cardboard or foam adds some strength to the door, but these doors are easily damaged because the exterior wooden skin on these doors is so thin. Many homeowners will find that they can save money by repairing these doors themselves instead of replacing them.
Cut away any rough edges or loose pieces of veneer from around the hole. Then sand away any remaining splinters or sharp edges.
Fill the hole with spray foam insulation. The cardboard "core" in the door will help restrict the foam so it only fills the area around the hole. This material expands fairly rapidly, so begin with a small amount and add more as needed.
Wait for the foam to dry completely. Use a razor blade to cut away excess foam from the surface of the door. Cut the foam inside the hole to just below the door's surface so you'll have room for patching and sanding.
Cover the hole with plasterboard joint compound. Apply this material with a putty knife. Extend the joint compound slightly past the edges of the hole. This will allow you to blend the edges of the hole with the rest of the door.
Allow the joint compound to dry completely and then sand it. Use a fine grit sandpaper to make the patched area smooth and even with the surrounding door surface. Wipe away sanding dust with a damp rag.
Finish the repair of a painted door by priming the patched area and painting the entire face of the door. Use a paint brush or roller to apply an acrylic latex paint.
Finish the repair of veneered or stained doors by mixing primer with a matching wood stain. Blend the stain and primer on a piece of scrap wood, then paint the patched area of the door with a small artist's brush. Try to match the existing grain and colour as closely as possible to minimise the appearance of the patch.