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Cement rendering is the application of a premixed layer of sand and cement to brick, cement, stone or mud brick. It is often textured, coloured or painted after application. It is generally used on exterior walls but can be used to feature an interior wall.
Depending on the 'look' required, rendering can be fine or coarse, textured or smooth, natural or coloured, pigmented or painted.
The cement rendering of brick, concrete and mud houses has been used for centuries to improve the appearance (and sometimes weather resistance) of exterior walls. It can be seen in different forms all over southern Europe. Different countries have their own style and traditional colours.
Different finishes can be created by using different tools such as trowels, sponges, or brushes. The art in traditional rendering is, (apart from getting the mix right), the appearance of the top coat. Different tradesmen will have different finishing styles and be able to produce different textures and decorative effects. Some of these special finishing effects may need to be created from a thin finishing 'top coat' or from a finishing wash.
There is also a wide variety of premixed renders for different situations. Some have a polymer additive to the traditional cement, lime and sand mix for enhanced water resistance, flexibility and adhesion.
Acrylic premixed renders have superior water resistance and strength. They can be used on a wider variety of surfaces, including concrete, cement blocks, and AAC concrete panelling. With the right preparation, they can be used on smoother surfaces like cement sheeting, new high tech polymer exterior cladding such as Uni-Base, and expanded Polystyrene. A few of these require activation with cement just prior to application.
Some of these premixed acrylic renders have a smoother complexion than traditional renders. There is also a wide variety of acrylic bound pigmented 'designer' finishing coats that can be applied over acrylic render. Depending upon the product, they can be rolled, trowelled or sponged on. A limited number can also be sprayed on. Various finishes, patterns and textures are possible such as sand, sandstone, marble, stone, stone chip, lime wash or clay like finishes. There are stipple, glistening finishes, and those with enhanced water resistance and anti fungal properties. Acrylic renders take only 2 days to dry and cure - much faster than the 28 days for traditional render.
To ensure adhesion, the surface to be rendered is initially hosed off to ensure it is free of any dirt and loose particles. Old paint or old render is scraped away. The surface is roughened to improve adhesion. For large areas vertical battens are fixed to the wall every 1 to 1.5 metres, to keep the render flat and even.
Cement render consists of 6 parts clean sharp fine sand, 1 part cement and 1 part lime. The lime makes the render more workable and reduces cracking when the render dries. Any general purpose cement can be used. Various additives can be added to the mix for increasing adhesion. Coarser sand is used as the base layer and slightly finer sand for the top layer.