While it may be tempting to rip down an old plaster-over-lath wall and replace it with drywall, often the plaster can be saved. Firm plaster that has popped off lath can be reattached with plaster washers and drywall screws. Soft, crumbly plaster should be removed down to the lath and out as far as the firmly attached areas in preparation for patching.
John Marshall starts by chiseling out the old plaster from between the lath and brushing away any loose dust and plaster crumbs.
(1) The plaster around the hole is scraped smooth with an angle plane or rough drywall screen. He brushes a bonding agent over the lath and all plaster being recoated, then covers all but the smallest cracks with fiberglass mesh tape.
To make lime putty for the patch, Marshall mixes hydrated dolomitic lime with water until it is the consistency of whole-fat yogurt, then slakes it for at least 20 minutes, leaving a thin layer of water on the surface to ensure even rehydration. The putty is formed into a ring on the mixing board and the bonding agent is poured into its center.
(2) Marshall sifts in several handfuls of gauging plaster to give the putty more body.
(3) He mixes everything with his trowel into a stiff dough, which he presses firmly onto the exposed lath and around the edges of the hole, leaving a slight depression for the final coat.
(4) To reinforce the patch, he cuts a sheet of fiberglass mesh to cover the hole and presses it into the wet plaster.
(5) After the first coat sets, Marshall mixes another batch of lime putty and gauging plaster (this time with less gauging so it's easier to work) and skims a thin final coat over the entire area. He then uses a sponge to touch up any surface imperfections. In 30 mintes the patch has set and the job is finished, without any need for sanding.
Plaster of Old
Most plaster used in the United States since the turn of the century is based on gypsum. Before then, walls were coated with lime-based plaster, which dates to the time of the pharaohs (it's on the walls of the tombs).
Limestone was burned (calcined) to drive off water and carbon dioxide. The resulting calcium oxide, called quicklime or lump lime, was slaked with water (rehydrated) in sand-lined pits. The mixture reacted explosively, but when the reaction subsided, creamy white lime putty remained. Mixed with sand and animal hair, it was applied over lath—horizontal strips of wood or straw. The first coat, or scratch coat, oozed between the laths to form "keys" that held the plaster in place. Two more layers, the brown coat and the finish coat, usually hairless, had to be "packed"—troweled again and again under pressure. Poorly packed plaster cracked as it dried; homeowners were warned to wait a year before painting.
Plaster made from calcined gypsum rock (calcium sulfate) forms a harder surface than lime without the bother of slaking and packing, but it sets almost immediately. In 1880, a retarding agent made of ground-up horns and hooves was found to slow setting. A quarter of the century later, vitually all the lime plaster in the United States had been replaced by gypsum plasters.
Still, there are some plasterers, like Rory Brennan of Putney, Vermont, who prefer to use lime, espcially for restorations. "Rigid gypsum plaster isn't compatible with the softer lime," Brennan says. He points out that lime takes centuries to harden: "Lime plaster is young at 100 years." he says.