This document discusses applying scratch coats for plastering. It explains that the sand/lime material used for scratch coats needs to be well-mixed so there are no large lumps, and it should be wet enough to spread easily but not too wet or it will sag. The proper tools for applying scratch coats are listed. A good scratch coat is shown that has been roughly straightened and contains a suitable mechanical key produced using a comb or wire scratcher.

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Applying scratch coats
Unit 122: Applying scratch coats to internal
backgrounds

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Applying a scratch coat

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Material for applying sand/lime scratch coats
The sand/lime material needs to be well mixed in the roller pan mixer
and wet enough to spread easily. The reason the material needs to be
well mixed is so that all large lumps of dry sand/lime mortar are
crushed and therefore do not create problems when applying the
material to the background.
Unmixed material will be dragged across the face of the work. This can
be time-consuming if every stroke of the trowel has to be corrected and
frustrating for the individual attempting to get a plumb surface ready to
receive the next coat.
If the material is too wet this can also create problems with the material
sagging if applied to thickly.
It is therefore important that the mix is prepared in advance of the class
and mixed at the correct consistency.

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Before starting
The work area should be prepared and include items such as:
• spot board and stand
• wet bucket and flat brush
• hawk and a trowel
• comb scratcher
• darby or feather edge
• shovel
• floor scraper
• brush
• wheelbarrow.

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Poor scratch coat
The mechanical key has to be suitable for the coat that will
follow the scratch coat. Therefore, when a further 10mm coat
of sand/lime or cement is to go on top of the scratch coat, it is
important that the mechanical key is deep enough to provide
a good key and bond.

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Good scratch coat
This is the first coat of three,
called the scratch or render coat.
It has been roughly straightened
and has a suitable key produced
with a comb scratcher or wire
scratcher. It is important when
using the comb scratcher not to
push through to the background.
The wire scratcher should break
through roughly 50% of the total
depth of scratch coat.

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Identify the coat of plaster
Question: Which coat of
plaster would follow this?

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Identify tools
Question: What tool is
being used?

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Identify tools
Handmade scratching tool Manufactured comb scratcher

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Any questions?