Plaster restoration for the owner of a historic home:
Many owners of historic homes have to deal with the problem of plaster that has been compromised by water or settling of the structure. Today there are several ways to deal with the issue but none of them are as good as repairing the plaster with original materials. Any attempt to mate plaster with gypsum compound will leave a joint that ismost likely to fail in short order.
Plaster walls are commonly found in older buildings. Historic plaster usually is installed over a substrate. This can vary but it is usually over wooden lath strips nailed to the wooden frame of the building. It can also be installed over masonry, such as brick and other masonry units or over a wire lathing instead or wood lath. In more modern homes (mostly those built in the 1950’s) the builders might substitute the traditional wooden lath or wire with Gypsum lath boards. These commonly have holes in them and are approximately 4 feet wide and 16 inches tall.
A brown coat of rough plaster was then installed over the substrate. This cement plaster material was often mixed with horse hair to give it strength. Some modern brown coat formulas incorporated asbestos in the mix, so one should be careful when working on plaster demolition. Generally the older the home, the less likely the brown coat contains asbestos. Today there is a readily available pre- mixed brown coat known as Structolite. This is a lightweight, Perlited, Gypsum plaster that is suitable for installation as a brown coat or even as a finish coat in some applications. The brown coat is troweled onto the substrate and rodded and gauged to an even surface over wall. This process requires a good deal of skill and also an eye for level and smoothness. The base coat is kept about an eighth of an inch shy of the desired surface. Once it is hard it is ready to accept the white coat of plaster.
It is not easy to find the material to do plaster work in this area any more. The essential ingredients are Finishing Lime and Gaugeing plaster. Finish Lime is sifted into water to make “lime putty”. Lime putty does not harden; it is simply lime that has been “Slaked” with water to absorb all of the water into the lime. It will stay in this state until mixed with the Guageing plaster. To do this one must make a doughnut of lime putty on a mixing board and fill the hole with water. The hardness of the finish plaster will be determined by the ratio of plaster to lime. About half and half is normally used. Sift the plaster into the water in the doughnut hole until it is absorbed fully then mix the entire batch together to make the plaster. When doing a large area, a retarder can be added to the plaster to allow for more working time. The plaster will begin to harden up in about fifteen minutes. Fortunately the plaster hardens gradually allowing the plasterer to work the material into a very flat and even surface. This is also a process that requires great skill and a good eye.
Plaster takes a long time to cure and it is loaded with lime. It is important to wait until completely dry before attempting to paint it. Use an all acrylic primer as these are meant to bond to masonry products.
Plaster can also be used to create decorative moldings and trim from the ordinary to the sublime. But That is a subject for another time.
written by James Fizmaurice Freelance decorative artist and designer, sculptor, with a background in construction and art.