Lime Base Coat Plaster for Scratch & Float Coats - Natural Fibre

Preparing the mix

• The plaster needs to be ‘knocked-up’ - a process of chopping, beating and turning which will release some water. Only the minimum amount of water should be added if required to make a workable mix, it should be used as stiff as possible.
• A mortar mill or forced action mixer is the ideal way to ‘knockup’ the lime plaster but this is not always possible. If a bell mixer is used, then it should be left turning for long enough to achieve a suitable consistency without adding water (20- 30 minutes).
• If water is required, it should be added CAUTIOUSLY.
• If hair is required, gently tease in towards the end of the ‘knocking-up’ process – avoid hair balls.

Application of the Plaster/Render

• Onto stone/brick/lath: Internally, it is usual to apply three coats – our Lime Base Coat Plaster is used for the scratch (first) and floating (second) coats, they should both be applied at about 8mm thick, the Top Coat Plaster (internal) should be applied in two thin coats totalling about 3mm. Externally, the process is the same for the first two coats but the Top Coat Plaster (external) should be applied in one coat of about 6mm.
• Apply the first coat of Lime Base Coat Plaster (scratch coat) with a steel float evenly. When it has firmed up, scratch the plaster (to about 3-4mm depth) diagonally using a wire scratcher. This could be the same day on brick or stone or, up to a week or more, on lath. This coat takes up any shrinkage and may crack. You need only worry if the plaster becomes detached from the background.
• Apply the next (floating) coat - as above but instead of scratching, you need to float the plaster whilst it is still soft enough to take the indent of your thumb. This will compact the plaster to avoid shrinkage cracks as well as flattening the wall. It should be carried out using either a straight grained (for uneven surfaces) or cross grained (for flat walls and ceilings) wooden float, not a plastic float. Floating is hard physical work, sometimes a little water sprayed onto the surface can help the process.
• After the floating coat has been floated and the plaster has firmed up, use a devil float to form the key for the finish coat, this is done by rubbing the surface of the float coat in small, circular motions to achieve a depths of not less than 2mm.
• The surface of the floating coat should be scraped down with the side of the trowel and then brushed to remove loose material whilst still green (holding moisture).