Plaster fireplace mantles, also written as mantels, are moulded, decorative accents to your fireplace. Plaster fireplace mantles are joined to wood blocking or a wood frame, often with wire mesh as a support, and screwed through plasterboard and into the wall studs. Over time, the plaster of a decorative plaster mantel can chip or crack due to stress on the plaster from the shifting of the house. Repair cracks, chips and small holes with a plaster mixture and a putty knife to halt further damage to the fireplace mantel.
Use a piece of fine-grit sandpaper to sand away any jagged or rough edges of cracks, holes or chips in the fireplace mantle plaster. If the crack is larger than 6 mm (1/4 inch) wide, slip the sandpaper inside of the crack or hole and lightly sand the interior edges of the crack.
Vacuum the plaster sanding residue from the fireplace mantle and from the cracks, chips or holes with a shop vacuum. Home improvement centres rent and sell workshop vacuum cleaners.
Mix 227 g (8 oz) of lime putty with 227 g (8 oz) of Plaster of Paris in a disposable container with a putty knife. Recycle clean margarine or other food tubs or plastic containers for an inexpensive source of disposable containers. Lime putty and Plaster of Paris are available at hardware and home improvement centres.
Add 1 tsp water and mix the putty and plaster with the putty knife. Continue adding 1 tsp of water and mixing the putty and plaster until the mixture has a creamlike consistency.
Scoop a 5p- to 50p-sized dollop of the putty/plaster mixture from the container with the flat edge of the putty knife.
Press the mixture into the hole, crack or chip. Overfill cracks and holes.
Use a damp rag or paper towel to wipe away excess repair mixture. For small cracks, chips and holes, wipe the excess away with a finger. Clean any repair mixture off anything else it may have dripped or fallen on. The mixture will set and be very difficult to remove if you wait to clean up once it is hard.
Allow the repair mixture to dry.
Sand the repaired areas with fine-grit sandpaper in a back and forth motion. Vacuum the sanding residue away with a workshop vacuum cleaner.
Finish the fireplace mantle to match the finish on the rest of the fireplace.