Traditional plaster can last for many years if properly preserved. The longevity of lime plaster is due to the fact that it cures continuously over long periods of time and its crystal make-up flexes under normal building movement. However, the ravages of time can take their toll. Earthquakes, water damage, large temperature variations and even a nearby construction project can move and crack a plastered wall. When this happens, repairing a traditional plaster wall or ceiling is often considerably lower than the cost of installing new plaster. It also helps to keep the authentic and historic look of older homes.
Although it would take years to accumulate the knowledge of a professional plasterer, there are patches and repairs that can be done by the do it your-selfer. First, you need to have the right material. It is common for a plasterer to use a ready mix base coat plaster (just add water) for the base coats (scratch and brown) in larger cracks. For the finish coat or the patching of small cracks and holes, use a high gauge lime putty which consists of 50% lime, 50% gauging plaster and water. You can buy gauging plaster in slow, medium or quick setting. For repairs, the quick setting gauging plaster is best because you will want the finish to set before the old plaster has a chance to absorb the moisture out of it and negate your repairs.
The first thing you need to do to repair a plaster wall or ceiling is to examine the area for structural soundness. If the lath is deteriorated, it must be removed and replaced. In this case, you will need to remove an area between studs so new lath can be attached to the existing studs for strength. If the lath is sound, then you should check the brown coat for firmness by scratching it with a trowel. If a lot of pressure is needed to make a mark, then the brown mortar is good and may be left and patched with a finish or putty coat. Sometimes, it is just the finish coat that comes loose from the base coat. To make this type of repair, paint a liquid bonding agent such as Bond-Weld on the area and refinish with a new quick setting lime putty coat.
To repair smaller cracks, it is recommended that you run a sharp pointed tool along the crack to remove loose particles and open the crack up slightly. Wet the crack to reduce absorption of water from the new patch. Fill the crack with a quick setting finish coat.
If the brown coat is deteriorated or the crack is large then you will need to remove loose plaster down to the lath. Generally, about 6” of plaster is removed on each side of a large crack. Place a strip of metal lath in the crack and hold it in place by nailing with lath nails into the sides of the old plaster and also into the existing lath. Reapply a new base coat up to the layer of the existing brown coat. Let the brown coat dry and apply the finish coat to match the existing plaster.
If the cracks are very large, or the area so damaged that it is not fit to patch, you will need to replaster the entire area. One way this can be done is by nailing furring strips (white pine or fur) at right angles to the joists. Attach lath (usually metal) and replaster. Fixing badly cracked areas this way is cleaner then removing all the old plaster and it adds to the insulation and fire protection quality of the wall or ceiling. Another way is to remove the old plaster, relath and apply new plaster However, either way, it is a large job and you may want to hire a professional for large areas such as this.