6708 13 l1_122_presentation_4

Level 1 Diploma in Plastering

© 2013 City and Guilds of London Institute. All rights reserved.

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PowerPoint presentation

Prepare backgrounds

Unit 122: Apply scratch coats to internal

backgrounds



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Preparation of backgrounds

‘Before anything else, preparation is

the key to success’

Alexander Graham Bell



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Prepare backgrounds

The previous statement was made by the man who invented the

telephone but can be applied to every plastering task.

The correct preparation of any

background is making sure that there

is a good bond between the

background and scratch coat. It is

arguably the most important part of

any plastering job and will certainly

extend the lifespan of the finished wall

or ceiling. If the preparation is poor,

the material will most certainly sheet

away from the background, leading to

cracks and possibly areas of the

plasterwork coming away from the

background.



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Background preparation

• The preparation starts with assessing the background to determine

the action to be taken.

• For example, if old plasterwork was perished, it would be removed

and the background would be brushed down and damped down

ready to receive the material.

• At the damping down stage, a bonding agent such as polyvinyl

alcohol (PVA) or styrene butadiene (SBR) can be used in order to

help to control the suction. You would not use a bonding agent if the

material being used is a lime/sand mortar.

• PVA and SBR form a barrier and help to control suction and allow

the materials to cure at the correct setting times.

• Traditional work which requires lime to be used as the main binder

will need to ‘breath’ and therefore no bonding agents such as PVA

and SBR should be used.



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Identify the background

What type of background is this and what type of

preparation would be required before a scratch coat was

applied?



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applied?



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applied?



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Examples of different types of backgrounds

Low suction – An example of a low suction background would be a

Stafford blue engineering brick (slide 5 pic). This type of background

would require expanding metal lath (EML), spatterdash or a modern

bonding agent applied to the wall before the scratch coat is applied.

Medium suction – A common clay brick used on internal party walls is

an example of a medium suction background (slide 6 pic). This type of

background would require brushing and damping down. A key could be

created by raking out the brickwork; however, a brush down to remove

heavy debris and a light dampening with a water spray should be

sufficient.

High suction – An example of a high suction background would be a

modern aerated lightweight block (slide 7 pic). These blocks have

excellent thermal properties due to their porous nature but are not as

strong as the backgrounds listed above. A 5:1 water to PVA mix could

be applied before the scratch coat in order to control the suction.



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Tools and equipment required for preparation

• Hard bristle brush/soft brush

• Shovel

• Wheelbarrow

• Water spray

• Chisels

• Feather edge

• Spirit level

• Claw hammer/lath hammer/scutch hammer

• Hop-ups/platforms for working at height

• Plumb line/string line



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Identify the following tools and equipment

Image courtesy of www.axminster.co.uk. Reproduced with permission.



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Image courtesy of www.axminster.co.uk. Reproduced with permission.



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Any questions?

6708 13 l1_122_presentation_4

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