The use of more advanced plastering materials such as gypsum-based plasters has relegated traditional lime-based plaster. Skilled contractors who specialized in lime-based finishes have become a rarity. Most often, they use plasterboard and skim plaster onto it.
This article will be discussing techniques and basic principles of the art of lime plastering. Most modern buildings nowadays are created using modern materials such as concrete, wood, or gypsum. Most plasterers are skilled in using these materials and sometimes have no idea how to use lime plasters.
Lime plaster has two distinct characteristics over modern plastering materials; they can slowly absorb carbon dioxide in the air, and they shrink as they dry. Lime construction has withstood the test of time and still visible in Rome’s ancient structures or the Great Pyramids of Egypt, which also used lime.
But how is lime prepared and mixed before applying it as plaster? Perhaps it’s better if we can see the process of plastering using lime. There are three steps in the lime plastering process: Scratch Coat, Floating Coat, and Finishing coat.
The Scratch Coat
The Scratch Coat is the first coat of plaster and involves creating a “scratch layer” in which are diamond-patterned plaster finish that will make way for the second coat. A mixture of 1 part lime putty to a 2 1/2 part course well-graded sand. This lime plaster layer will create a slightly coursed surfaced area so the second plaster coat can adhere to it.
Hair or natural fibre is sometimes added into the mix to strengthen the plaster, especially if it is placed on wooded walls or ceilings. If its place on brick walls, the hair or wool is unnecessary since the bricks will provide the strength and rigidity. According to plasterer Hobart, the scratch coat should not be more than 15mm thick. The scratching should form diagonal straight lines will intersect each other, creating a diamond pattern plaster finish.
The surface should still retain a little moisture before applying the next coat. A water spray can be used to dampen the fish if in case it dried up quickly,
The Floating Coat
Following the scratch, coat is the floating coat, consisting of a mixture of 1 part of lime putty and three parts coarse well-graded sand. It is also called a straightening coat and should not exceed a thickness of 15mm.
The floating coat will cover the diamond-patterned scratch coat with a flatter and smoother finish. Wooden blocks, which are known as “dots,” are used to make the plaster more levelled for a refined lime plaster finish. The surface will be compacted, sprinkled with water, and regularly brushed to prevent the lime plaster from quickly drying up. This process will be repeated, and then the coat will be left for around a week before proceeding to the last coat.
The Finishing Coat
The last coat is the thinnest, which can be between 2mm to 5 mm in thickness. It is the riches mixture of 3 parts lime to 1 part fine sharp sand. More lime will mean a smoother finish, but it can easily be adjusted according to the plaster’s required quality and appearance.
The finishing coat will require very thin layers, which are applied only when the previous layer is dry. To have a smoother surface, a combination of wood or sponge can be used depending on the design’s texture. Plaster of Paris is sometimes used as the last layer, especially if the surface needs to be very smooth and free of blemishes. The plasterer needs to spray water once in a while to prevent the plaster from drying up too fast. The newly repaired lime plaster should match the colour, texture, and look of the old plaster surface.