Cornices are mostly found in old Victorian homes that used cornices as a decorative border around the ceiling to enhance the beauty of a home. Though the moldings are exquisite, they are rarely found fully intact. Modern versions of those Victorian embellishments are implemented into newer homes. Repairing cornices, whether they are old or new, depends on the detail of the cornice. The more elaborate the design, the more difficult it will be to restore.
Clean the damaged area by removing any loose particles of plaster.
Cut a styrofoam piece in approximately the same size and shape of the missing cornice section. Do not to cut a piece that will stick out from the rest of the cornice's plane.
Apply plasterboard adhesive into the gap with a taping knife.
Place the cut-out styrofoam pieces onto the adhesive.
Apply another coat of plasterboard adhesive on top of the styrofoam. Smooth it out with the taping knife. Do not apply adhesive so that it sticks beyond the plane of the cornice. Let it dry for 24 hours.
Sand down any parts of adhesive that stick out over the cornice.
Mix plaster mix according to the manufacturer's directions.
Apply the plaster onto the adhesive with a taping knife, mimicking the rest of the cornice's shape and dimension. Apply another layer of plaster to achieve a more defined cornice shape.
Sand the area after it has dried. Wipe clean with a damp sponge. Paint over the repaired section.