By aboderenovationfaversham, Jun 26 2015 09:18AM
Lime Rendering on to Brick or Stone Walls Application Guide
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.
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.
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.
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.
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 us.
4. First Coat\ Scratch Coat
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 and treat wooden lintels with preservative and counter batten with oak lath if they are to be plastered over.
Apply one hand harled coat of Lime 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.
5. Second and Subsequent coats
Apply sufficient coats of haired or unhaired lime putty mortar, 3:1 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.
6. Optional Final Plaster 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.
Our 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.
for an Example Specification per square metre on to a rough undulating surface
Scratch coat 3/1 haired lime putty mortar, 30kg per m2 (15mm)
Float coat 3/1 unhaired lime putty mortar, 20kg per m2 (10mm)
Two top coats of 3/2 lime putty plaster, totalling 6kg per m2 (3mm)
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.