This document discusses materials and procedures for applying scratch coats to internal backgrounds. It identifies common materials used in scratch coats like sand, cement, lime, and additives. These materials are mixed together with tools like cement mixers or buckets. Scratch coats are the first of three coats applied and are used to even out uneven backgrounds before applying float and setting coats. The document provides step-by-step instructions for thoroughly mixing materials like first adding half the water, then sand and cement, and finishing with the remaining water. Personal protective equipment should be worn when mixing.

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Mixing materials
Unit 122: Applying scratch coats to internal
backgrounds

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Mixing materials
Below are some of the materials that can be used in a scratch
coat:
• sand
• cement
• lime (hydrated)
• additives such as waterproofer.
It is important to mention that all of the above materials are not
required for every scratch coat. The contents of the mix will
depend on the particular job and background. However, there
will always need to be a binder and an aggregate in every mix (ie
sand and cement).

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Identify some materials used in scratch coats

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Material descriptions
Sharp sand – This is an inert material known as an aggregate. One of the
qualities of sharp sand is its different aggregate sizes, making it a well graded
sand.
Hydrated lime – This is mainly calcium carbonate or limestone that has been
heated up to remove the carbon dioxide, turning it into a calcium oxide known
as lump lime. Slaking lump lime and driving off the excess water will produce a
powdered hydrated lime.
Cement – Probably the most common cement is Portland cement, so called
because of its similarities to Portland stone when it cures. Its two main
ingredients are limestone and silica, with other ingredients used in the
manufacture process, such as clay, blast furnace slag and iron ore.
Plasticiser – This is a soluble organic sulphonate which, when added to sand
and cement mixes, can improve the workability and the mortar’s durability in
freezing cold conditions.

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Scratch coat
Scratch coat is a term used to describe the first of three
coats on internal work, followed by float coat and setting
coat.
Scratch coat > Floating coat > Setting coat
• The traditional term used instead of scratch coat is
render coat.
• Scratch/render coats are used when there are composite
backgrounds or uneven backgrounds. The scratch coat
will even out the suction for the next coat.
• Scratch coats should be applied to an approximate
thickness of 9 to 12mm.

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Identify the following equipment
Image courtesy of Multi Marque Production Engineering Ltd. Reproduced with permission.

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Identify the following equipment
Image courtesy of www.axminster.co.uk. Reproduced with permission.

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Identify tools
What other tools or equipment can be used to mix
materials for a scratch coat?

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Identify tools and equipment
Image courtesy of www.axminster.co.uk. Reproduced with permission.

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Mixing materials
• When using any of the equipment listed above, it is important
that the materials are mixed thoroughly.
• When mixing quantities that require several mixes, all the
mixes should have equal measures and be consistent in their
texture and appearance.
• Equal measures can be obtained by weight, ie 25kg bagged
materials, or by using buckets to ratio equal volumes.
• Care should be taken with the volume of water being added
and additives such as plasticiser. The total volume of water to
be added will vary depending on the water content in the sand
and the amount of plasticiser/lime being used.

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Step-by-step guide
1. Add half of the water and additives (if additives are required)
2. Add the sand
3. Add the cement. Lime can also be added at this stage if no
plasticiser is being used
4. Continue to add the rest of the water when required. The
water content can vary slightly depending on the water content
of the sand that is being used and if a liquid or powder plasticiser
is being added.
5. Allow enough time for all materials to be mixed thoroughly (5–
10 minutes)
The appropriate PPE, goggles, gloves and dust mask should be
used when mixing materials such as cement, hydrated lime, sand
and additives

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Any questions?