The plaster that we apply to our internal walls is applied in various different coats or layers on walls or lath and the type of application depends on the number of these coats. One coat is reserved for course, cheap work that does not a decorative feel. Outhouses and brick sheds are often subject to only one coat. Two coat plastering uses one coarse base coat topped by a smooth layer using a trowel and brushed smooth. Most modern houses are plastered using this method, with the two coats being applied to plaster boards. The type of application that was most used in many older domestic properties is three coat and this article describes the process of three coat plastering.
The first coat consists of a layer of well-haired coarse plaster, of approximately one inch in thickness, which is applied with a laying trowel in a diagonal pattern. Each trowel full overlaps the one applied previously. If this scratch coating is to be applied to lath, the plaster should be flexible enough to be worked through the spaces between the laths to provide a key, but it also needs to be sufficiently sturdy to ensure that it does not fall off. Whilst the plaster remains soft, it is scratched to provide a good key for the second coat. This layer is usually made from a mix of 2 x sand and 1 x lime, which has around a pound of hair mixed into it for every two or three cubic feet of mortar. Clean water is then added to the mix in an amount that allows the mix to stick to the trowel without dropping off.
The second layer, which is also known as the “floating coat” is between a half and a quarter inches thick. Laying the second coat involves four procedures. The first is to prepare the screeds, which are narrow plaster strips which form a guide for the laying of the second coat, to ensure a perfectly horizontal or vertical surface. Secondly, the spaces between the screeds are filled with coarse plaster. Thirdly, the floating coat is scoured, binding and hardening the plaster and preventing it from cracking. The plaster is scoured with a hand float and water and this is usually done two or three times. To add a key, the surface is then scrubbed with a wire brush or nail float. When this has been completed the third coat can be applied.
The last of the three layers is the finishing coat. This is the thinnest of all three layers, being around one eighth of an inch. It provides a smooth, cosmetically attractive finish which is ready for painting or papering once it has dried out fully. The final coat consists of lime putty, which is mixed with water and left to stand to give it a soft pastey consistency. Many plasterers use a water spray to help them to ensure the smoothest of finishes is achieved for the top coat.
The application of a three coat plaster surface is highly skilled. Although it may seem quite complicated, for an experienced plasterer it is all in a day’s work. Although it is not a method that is normally used in new houses, in older properties, which have irregular, uneven walls, it may be the most appropriate method to employ when the property needs to be re-plastered as part of a renovation project.
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