The Use of Base Coat & Finish Plasters – TAN9
Posted 4th March 2020
Non-hydraulic lime putty internal plasters are soft, porous and permeable. These are the materials used historically to plaster traditional buildings and are therefore normally the appropriate materials for re-plastering and plaster repairs. Modern Natural Hydraulic Limes (RJ1.05) are being increasingly used for re-plastering traditional buildings but these modern materials produce stronger, less porous and less permeable plasters than the traditional material.
STANDARD PREMIXED PLASTERS:
SPH5 Basecoat plaster with fibres added at a rate equivalent to 2½ kgs animal hair per tonne
SPH8 Basecoat plaster with fibres added at a rate equivalent to 4 kgs animal hair per tonne
SPUH Basecoat plaster without fibres
SFF Fine Finish Plaster
SMF Medium Finish Plaster
Lime putty plasters are supplied ready-mixed in a ‘ready-to-use’ consistency, but can stiffen during storage. Improved ‘workability’ can be achieved by re-mixing in conventional plaster mixer, or by whisking in a tub or bath. Avoid the addition of water, as this will increase the likelihood of shrinkage. Lime putty plasters do not perform as modern gypsum, hydraulic lime or cement-based plasters.
Do not gauge with cement. Pozzolanic materials (RJ 7.01) are acceptable additives if the speed of set or strength is a concern in a particular application or location. Seek Technical advice if gauging with gypsum or hydraulic lime is specified.
Lime putty plasters must be stored in a frost-free environment. Do not use in extremes of heat and cold. Lime plasters must be protected from drying too quickly by regular mist-spraying with water which aids carbonation. Heating systems must be turned down and de-humidifiers should not be used.
Lime putty plaster is not suitable for use on dense concrete block, plasterboard or EML (Expanded Metal Lathing). Riven (hand-split) oak or chestnut lath (RJ7.10) is the correct lathing.
Lime putty plastering is a specialist trade. Successful use of base-coat plaster depends on application in maximum 10mm coats and control of the drying process. On timber laths it is crucial that the ‘pricking-up’ coat (use SPH8) is pushed through the gaps to form good nibs behind as the plaster ‘hangs’ from the nibs and does not adhere well to the laths.
Basecoat plasters are finished with fine or medium finish plaster, often applied in two coats ‘wet-on-wet’ and then decorated with a permeable paint such as Soft Distemper (RJ 4.01), Casein Distemper (RJ 4.02), Pure Limewash (RJ3.01) or Casein Limewash (RJ3.02).
Disclaimer: The information provided in this advice sheet and all technical advice is for guidance and is given in good faith but without warranty, since the site conditions and care and skill of application are beyond our control. We can accept no liability for the performance of our products, beyond the value of the material supplied. This does not affect your statutory rights.