How to Fill a Hollow Bifold Door

Use your utility knife to cut a clean edge around the hole in your hollow bifold door. Stuff the hole with ordinary paper, rolled into a ball. The idea is to fill as much of the interior void as possible with scrap material.

Use your can of spray foam insulation to fill the rest of the hole. Hold the nozzle of the can up to the hole, and spray insulation into the void. Allow the insulation to dry according to the manufacturer's instructions. The foam will expand substantially during the drying process, and, in so doing, it will fill the hole inside the door completely.

Use your utility knife, or a straight razor blade to cut the extra foam insulation from the door. Run your blade flat against the door when cutting, so the foam filler ends up flush with the surface.

Use a plaster spatula to apply joint compound over the foam-filled hole. The idea is to eliminate any subtle changes in surface height, or other imperfections present in your patch job. Allow the compound to dry according to the manufacturer's instructions. Use fine-grit sandpaper to smooth over the plastered surface until it is indistinguishable from the rest of the door.

Use a paintbrush to apply a coat of primer to the entire door. Allow the primer to dry according to the manufacturer's instructions. Use your clean paintbrush to apply a coat of paint to the entire door. Allow the paint to dry, then add a second coat if needed.