What's the best plaster to use?
What's the best course for you?
The following information may help you choose to use either a conventional plaster or lime plaster:
Conventional Plaster defined
Backing coat: sand & cement, or gypsum based such as 'Hardwall' or 'Browning'
Skim Finish Coat: gypsum based, Multi-Finish or Thistle Board Finish.
Lime Plaster defined
Backing Coat: Lime mixed with sand
Skim Finish Coat: Lime mixed with sand (slightly stronger mix, finer sand)
Conventional plaster is:
Lime Plaster is
A rough guide to which plaster to use
Consider using Conventional plaster if:
Consider using lime if:
Lime Plaster is classified as either:
Hydraulic or Non-Hydraulic
The main difference
Hydraulic lime will set when wet, whereas non-hydraulic will not set when kept moist.
A non-hydraulic lime in exposed conditions externally (without the presence of an additive such as pozzolans) may not withstand the elements as well as a hydraulic lime. Both types of lime harden through a process of carbonation however the hydraulic lime also has a chemical set as a result of impurities present within and so hardens more. But the harder it is the less flexible, so a balance is sometimes needed.
Non-Hydraulic lime - available as a lime putty (flexible and able to withstand the most movement) or hydrated lime (white powder) from builders merchants.
Hydraulic lime is available in bags in different strengths (NHL2, NHL3.5, NHL5). The stronger it is the less flexible.