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How is Lime Plaster mix when plastering walls?

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. 

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When to Replaster Walls and Ceilings

Traditional plastering has been proven to be extraordinarily tough and flexible. If you have seen old houses or often called period homes, you will be amazed at how these plasters are still intact even after decades or even centuries of use. It’s a testament to the plasters’ toughness and the quality of work done by these magnificent builders. 

Original plasters have always been one of the highlights of these old buildings, which add to their charm and nostalgia. Old plasters made of lime and clay are soft, flexible, and has a unique look and character. But of course, through years of use and exposure to elements, this plasters age eventually and may need to be fixed and restored.  

Fixing old plasters can be relatively easy, but sometimes it gets complicated due to the unfamiliarity of contractors with the materials. These contractors will most often use the wrong techniques and materials, causing damage to the old plasters. Old plastering needs to be fixed properly, but some contractors seal the old plaster’s moisture, resulting in decay and may lead to them eventually falling off. So homeowners should look for contractors familiar with this type of applications to repair and restore them properly. 

When do you know it’s time to replace or repair plasters?

Traditional plastering is very sturdy and will last for a long time, but it may need some form of repair or renovations over the years to make it last longer. The first sign is cracking; it may start with hairline cracks and eventually grow broader and more extended. It is essential to have it fixed while still small to not progress to a more significant gap that would longer resolve. Another initial sign is staining, you will notice a slight patch of discolouration, and then it becomes darker and more prominent as it progresses. 

When the initial signs are not fixed right away, more damage appears. The plasters will start crumbling and flaking; you will notice pieces of plaster on the floor. At this stage, it’s essential to call contractors who specialize in traditional plastering to assess the damage and immediately make repairs. 

Soon after, you will see large areas of the plasters falling off. Visible mechanical damage becomes very apparent. It is essential that the plastering does not reach this point, so early intervention is critical. Waiting will make matters worse and may become more expensive to fix. 

Make sure the contractors are familiar with traditional plastering. If possible, the same materials should be used for the repairs. But in cases where the materials are no longer available, viable alternatives should be used. These materials should have similar characteristics to the original plaster. The use of plasterboards should be avoided because it will ruin the plasters’ look and make it out of place. For traditional plastering, it is always advisable to use a lime plaster

Apparent signs of plaster damage like cracks or falling plasters are easy to spot. But, You should also watch for other signs of wear in the plaster finish. Try to tap on the walls for hollow sounds that may signal internal damage to the plastering. An obvious sign of damage is the sagging of ceilings, especially if there are signs of water stains. It could be a sign of a water leak, which can rapidly damage the area. 

Some problems with plastering is often a result of underlying structural issues, water pipe leakages, and roof leaks and should also be addressed so that the problems do not come back. But for many issues with the plastering, whether it’s minor or significant, it’s better to consult professional contractors. Do it yourself would also be an option. But if the cracks or damage appear again, it might be a sign that there are other underlying reasons. At this point, it’s best to call experts to find out the source of the problem.