Horsehair plaster is a type of plaster made from a mixture of lime, aggregate, water, and animal hair (horsehair being very commonly used), which is then applied onto laths (thin flat strips of wood nailed to the studs.) The mixture created a gritty, textured look and feel that has been used as far back as the 1800s. It is hard to find this type of plaster in newer, more modern homes since this technique has long lost its popularity due to its predisposition to severe cracking and crumbling with age. However, some older modeled homes still have horsehair plastered walls and ceilings, and some homeowners enjoy the antiqued look of horsehair walls, regardless of the superficial cracking that comes with them. In actuality, small cracks usually do not represent any real architectural damage and can be easily repaired using the right tools and following the proper preparation procedures.
When performing this type of repair work, you must take extreme precautions due to the amount of loose dust, dirt, and old plaster you will be releasing into the air. Older plaster compounds like horsehair plaster and the type of paint covering it may contain many harmful chemicals, such as lead. Because of this, no children or pets should be allowed in the area while work is being done, as it is potentially dangerous to breathe in the loose particles that will be floating in the air. Protect yourself as well before embarking on this type of project by making sure you wear your dust mask, safety goggles, and work gloves at all times during the repair process.
Step 1 - Prepare Room
Remove any furniture from the room you will be working in and put on your dust mask, safety goggles, and work gloves to protect against loose plaster and dust particles. Lay down a plastic tarp or drop cloth to protect your flooring. Using a sharp pointy tool, chip out any of the loose plaster you can find in each crack you are trying to repair. Avoid using undue pressure, as this could lead to unnecessary damage and the need for further repairs.
Step 2 - Prepare Crack for Compound
Using the hose attachment on your vacuum, suction out any loose debris from the open cracks in your plaster. When you're done, mist the area lightly with water to reset the plaster. Allow the area to fully dry before proceeding to the next step.
Step 3 - Apply Compound
Beware of using any patching product that contains gypsum for skim-coat repairs, as it is not chemically compatible with lime-based plasters. Drywall materials are not suited for lime plaster repairs, especially not in a historic restoration setting, and have a high risk of premature or rapid failures. Lime plaster has great flexibility—which is why it's lasted on the walls as long as it has done—with the huge downside, however, that it has a very long cure period.
So with this being said, generously fill the cracked plaster with a quality lime compound. Using the putty knife, smooth the area and scrape off any excess compound before allowing it to fully dry.
Allow at least 24 hours to pass before beginning the sanding process. Making sure your protective gear is once again in place, use a firm sanding pressure to further smooth out the repaired plaster, making clean swipes with the sanding paper. Once you are finished sanding, wipe the dried compound clean with a damp sponge. After checking on the appropriate waiting time before applying the paint, prime the repaired area neatly, allowing the primer to dry before applying a coat of paint for the finished result.