Plastering

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Lime putty plasters offer advantages over cement based mortars and pure gypsum plasters for the internal plastering of traditional properties, especially when decorated with a traditional limewash or other natural, breathable paint.

  • their porosity allows the structure to “breathe”.
  • they can accommodate general movement better.
  • their self-healing nature reduces cracking problems.
  • they can reduce condensation problems

As with all lime putty based materials the best outcome requires patience and careful control of drying and suction, the reward being a good looking and long lasting plaster.

We also supply a range of earth plasters for use in traditional properties and new build. They are designed to be used over cob, clay boards and reed mat.

Before starting any work, always check that the building is not listed, if it is contact the local authority conservation officer to discuss the project.

Preparation:

Any existing plaster and paint should be removed. If there are sound lime mortars these should ideally be left to help preserve the buildings history and it clearly reduces material costs. Care must be taken to ensure that the structure isn’t damaged. Look out for very thick patches of plaster that are effectively load bearing. It may be necessary to plaster on top rather than risk rebuilding an area.

Damping:

It is very important to control suction from the background material (substrate) by damping with water before applying each coat of plaster, especially onto cob or porous brick. Although control of suction is important, so is surface tension so any damping down must be allowed to soak in and not be sitting on the surface when the plaster is applied.

Premixing:

Generally, lime putty mortars and plasters benefit from being pre-mixed for a minimum of a couple of weeks and then “knocked up” again prior to use to plasticise them – this reduces shrinkage in the plaster. The pre-mixed and ready to use materials can be purchased from Mike Wye & Associates Ltd. or our outlets.

Example Specification on to solid walls:

  • dub out any deep holes in the wall with a haired lime putty mortar, rebuilding defects using like for like materials such as cob blocks, cob bricks or stone.
  • treat wooden lintels with preservative and counter batten with oak lath if they are to be plastered over.
  • apply one hand harled coat of Secil Consolidation Mortar to provide a key to cob or brick. With stone this is a matter of preference depending on the friability and size of stone and joint if this stage is ignored.
  • apply sufficient coats of haired lime putty mortar, to smooth the contours of the wall, the last float coat may benefit from being unhaired as then there is no chance of hair coming through the top coat.
  • apply a top coat of our lime-rich, 3/2 lime putty plaster, based on a very fine sand and lime putty in either one or two applications. For the very smoothest of textures, the second coat should be a 1mm skim of our Regency lime putty plaster.

Gauging:

If there is a high residual level of moisture in a wall that cannot be eradicated (eg a high external ground level) it may be necessary to sandwich a waterproof barrier between the coats of lime mortar. We supply a waterproofing slurry for this purpose. Backing coats of lime mortar can be gauged with Argical M1000 or NHL5 (eminently hydraulic lime) to get an hydraulic set. Argical M1000 or natural hydraulic lime (NHL) can be added to the top coat plaster to ensure extra durability where knocks are expected, 10% Argical or NHL5 by volume.

Plaster Skim:

Our Regency or 3/2 fine lime plaster is ideal for a thin skim over a wide variety of backgrounds such as plasterboard, blocks and mixtures of old and new plaster, old paint etc. For plasterboard it will be necessary to apply a special bonding coat DG27. For bare plasterboard, it is always necessary to scrim the joints as is usual practice, normally this would be applied with a very thin coat of gypsum although lime wall finish smooth could be used. Failure to carry this out may result in cracking at the joints. Our lime plaster 3/2 can then be applied in one or two very thin coats or Regency plaster for the second coat for the very finest of finishes.

Quantities:

for an Example Specification per square metre on to a rough undulating surface
3 mm scat coat of Secil Consolidation Mortar 5 kg/m²
15 mm scratch coat haired lime putty mortar, 30 kg/m²
10 mm float coat unhaired lime putty mortar, 20 kg/m²

3 mm two top coats of 3/2 lime putty plaster, totalling 6 kg/m²

Safety:

Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.

Our advice and information are given in good faith. It’s important that users satisfy themselves that they’ve chosen an appropriate product and have a suitably skilled workforce.

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