Toni’s Lime Plastering Guide
Below is a short description of a plastering method I have successfully used on multiple projects.
Preparation
All Plaster mixes and ratios are using a hydrated lime putty and clean sharp plaster sand. Preferably marble sand due to the chemical and mechanical bond, however quartz sand also offers great results.
The purpose of a plaster is to add a layer of protection to the house shell which means the plaster must be applied with attention to ensure correct adhesion to the underlying surfaces as well as an even application across the entire wall/ceiling surface.
Lime plasters must always be applied onto a clean surface in layers (usually 3 layers, however on walls which are even and don’t require reinforcing 2 layers can be applied, but not less than 2). Layers must be applied onto a damp surface, so that the plaster is able to bond to the underlying surface and doesn’t lose moisture too quickly.
If the HempLime surface has holes or loose bits, these should be filled by hand with a HESwall mix (HempLime mix with extra Lime putty added). The mix must be workable and sticky enough to make a sculptural material to plug holes and stick to the underlying surface.
Lime plaster hardens through carbonation and the plaster should not dry out too rapidly which means it may need to be protected from intense sun, wind or heavy rain during its initial drying phase (for the first two days). The initial drying time will be quick and subsequent layers can usually be applied to the base coat within one to two days of first application (depending on temperature and moisture levels as well as thickness of coat)
The carbonation/curing continues for months and carbonation only occurs while there is still a level of moisture in the plaster (for the CO2 to displace the water) and therefore it is important that the plaster cures slowly.
No cement must ever be used in the lime plaster.
NHL or Pozzolans can be added if the plastering is being done outside in a wet time of the year where initial drying times need to be faster than one to two days (if heavy rain is expected.
Ensure that the surfaces are ready and that all necessary plastering tools and equipment are at hand to begin plastering before starting with mixing.
Lime plaster should not be applied in temperatures below 5 degrees Celsius
Plastering
The thickness of plaster layer should generally not exceed 4 times the largest grain size. E.g. A sand of 0-3mm grain size should have a maximum layer thickness 12mm.
The first layer is the base coat or leveling coat for uneven surfaces. The base coat is the foundation of the subsequent layers which means it needs to bond well to the existing surface. The application of the subsequent layer will depend on the drying time (temperature, humidity and application thickness) of the base coat.
Subsequent layers must only be applied once the plaster has set enough not to deform but is fresh enough to bond chemically.
The base coat may be an uneven thickness and therefore may be left to dry for a few days before subsequent layers are applied. The base coat surface should be roughened/scratched while still fresh (using a devil float or similar) to provide a mechanical bond for the subsequent layer. if left to dry, the surface must be wetted before application of subsequent layers.
All reinforcing is done in the first layer. Reinforcing in the form of long (cut to min.10cm) strands of hairs or fibers mixed into the plaster or as netting embedded into the fresh plaster layer. Reinforcing is recommended over all existing cracks as well as over diagonals above door and window openings. A minimum of 30cm wide reinforcing is to be applied over these areas.
Finish layers can be added in thicknesses of 5mm to 1mm or less and can be finished with a wooden float, sponge float or metal trowel to a grainy or smooth finish.
Be patient in the application, don’t try and finish the surface too soon, allow the plaster to set slightly for a better surface finish.
Mix Ratios by volume:
Plastering
Ratio = Lime Putty: Sand
• 10-15mm Base coat: 1: 3
• 2-5mm Finishing coats: 1: 2
• 1mm Super finish: 1:1
• Insulative Plaster 3cm: HempLime mix with additional Lime putty
By wet weather additives such as HESplus, Pozzolan or NHL can be added at approx. 5% of lime putty volume.
Lime Putty
Ratio = hydrated lime powder: Water
• 25kg: 40L
Add water into a 200 L drum and then the lime powder, mix well for at least 5 mins, cover and allow to soak for at least 18 hours.
HES mix
Ratio = Hemp Hurds: Hydrated Lime Powder: HESplus: Water
• Standard Building mix: 20kg: 25kg: 3kg: 43L