How To Do Lime Plastering
Lime plastering is an eco-friendly, traditional finish for interior or exterior walls. It was traditionally used over lower quality building materials, like rubble stone or porous brick, or on walls that faced a constant, driving wind.
Lime plaster has the unique quality of being able to absorb and hold rain, allowing it to evaporate back into the air rather than allowing it to soak through the wall. It is a common finish for small cottages, and modern eco-friendly builders use it as an ideal finish for hay bale or earth building.
WARNING: Lime plaster contains alkaline materials that easily burn skin and eyes. Wear protective clothing, gloves and safety glasses while making or handling lime plaster, and remove any bystanders from the area before working.
Step One – Prepare the Wall
Before beginning with lime plastering, prepare the wall to be finished. If you are working with a stone or brick finish, remove old, decayed or hollow lime plaster. Remove dust and loose material, but don’t remove old plaster only because it seems soft. Lime plaster is softer than cement-filled modern plasters.
If you are doing restoration work, have an expert match your old plaster for a compatible mix.
If you are plastering on wood laths, make sure the old plaster is smooth and any broken areas or lumps are removed. Split (not sawed) river oak or chestnut make the best laths. Leave an 8 to 10 mm space between the laths, approximately the width of a finger.
Secure laths firmly. Metal laths are quicker and less expensive to use but can be more difficult to plaster because they are slippery.
When the walls are ready, spray them with clean water using a spray attachment on a hose. Don’t overspray as this will cause puddles and runs.
Allow the water to soak into the wall, then spray it again until the wall is completely damp. The wall, including any stone or laths, should be thoroughly damp when you apply plaster.
Step Two – Mix the Plaster
Lime plaster is made by using either a pre-mixed lime putty or hydrated lime that must be soaked for no less than 16 hours before use. Many experts suggest waiting 3 months before using a lime putty or hydrated lime to be sure it is properly slaked or ready to use.
Mix the lime with sand to get the final mixture, using three times as much sand as lime to get the right proportions.
Then add gauging material—animal hair, gauging plaster or Keene’s cement—according to directions. Mix only as much plaster as you are able to work with before it completes the initial set.
Lime plaster is thick and plastic-like. If the mortar is too thick, it will not stick smoothly. If it is too thin, it will go on nicely at first, but then start to sag.
Step Three – Apply the Plaster
Applying lime plaster well takes experience. Practice on small areas. Use a thinner fill coat first to fill in hollows, and then apply the first coat. Lime plaster will follow wall contours and does not need to look flat and smooth.
Push (render) plaster between laths to provide a solid surface. You will need to use solid pressure to work it into all the gaps. Use a gauging trowel for the initial coat if it helps with the process.
Once the scratch coat is applied, apply additional coats. Each additional coat should be mixed with less aggregate so they will go on more smoothly and with less pressure. Keep the plaster and the wall damp as you work
Step Four – Allow to Cure
Although lime plaster will feel hard to the touch in hours, it will not be cured for weeks. Give the wall time to cure thoroughly before testing the plaster or moving items into the area. Section off the area with tape or other items to keep children or pets from touching the wall.