Prepare Background Surfaces
Prepare Background Surfaces and Plaster Materials
Prepare Backgrounds
Develop an understanding of different types of suction on backgrounds as well
as the mechanical key they will have.
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Prepare Backgrounds
Aims:
To understand the different backgrounds to which
plaster is applied.
Objectives:
To know and recognise different backgrounds.
To know how to prepare different backgrounds.
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Prepare Backgrounds
What is a background ?
A background is any surface which plaster / render can be applied to.
Examples of backgrounds
Brickwork
Blockwork
Concrete
No Background No Plastering
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Prepare Backgrounds
Introduction:
What do you know about backgrounds ?
Try answer the following questions
1 What is suction
2 How can suction be controlled
3 How can suction be created
4 What is mechanical key
5 What is a bonding agent
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Prepare Backgrounds
Background:
If a background is porous it will absorb water like a sponge.
If a background is dense it will absorb little or no water.
This is called SUCTION
The more water the background absorbs the higher the
suction rating
The less water a background absorbs the lower the suction
rating
Question
What effect will this have on the plaster applied?
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Prepare Backgrounds
Backgrounds:
Gypsum plasters rely on retaining some of the water used to
mix them until the setting process is complete, if too much of
the water is absorbed by the background the plaster will
shrink and crack and lose adhesion with the background
causing bond failure.
Suction needs to be controlled.
How can you tell if a background is HIGH, MEDIUM or LOW
SUCTION?
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Prepare Backgrounds
Suction:
To establish how much suction a background has is quite simple. It’s as simple
as splashing some water onto the background and watching how quickly the
water is absorbed, the quicker the water is absorbed the higher the suction.
Over time and with practice this will become quite a good indicator to you has to
how much suction a background has.
Try it !
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Prepare Backgrounds
Background Preparation:
High Suction
Backgrounds
High suction backgrounds:
Bonding agents are used to seal the background and control
the suction to leave a surface suitable for plastering.
The most widely used are PVA, Thistle Gypprime and SBR. It
is vital that the manufactures instructions are followed when
using these products to avoid problems in the future.
Thermalite Blocks
Easily applied using a roller
Porous Bricks
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Prepare Backgrounds
Background Preparation:
Low Suction Backgrounds:
Because plaster relies on some suction and in most cases
some form of key this type of background needs the most
treatment.
Dependant on the type of background (tile, concrete) the
easiest treatment is to coat the surface with a bonding agent
such as Thistle bond it, which creates a key and creates an
even suction throughout the background.
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Prepare Backgrounds
Background Preparation:
Different backgrounds require different treatments before we
can apply any plaster.
Treatments vary from:
Damping down
Bonding agents
Scratch coats
Hacking
Raking out of joints
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Prepare Backgrounds
Creating Key:
Mechanical Key
Hacking:
This can be done using mechanical or manual means and is
done to provide a key for the plaster bond to a low suction
background.
Raking out joints:
Again this can be done both mechanically or manually.
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Prepare Backgrounds
Creating Key:
Mechanical Key
Scratch Coats:
Applied using various materials depending on circumstances,
can be used to create key, suction or to even out suction.
Bonding agent:
Bonding agents are easily applied using a roller, and are
used to seal the surface and with some bonding agents to
create key as well.
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Prepare Backgrounds
Creating Key:
Mechanical Key:
Expanded Metal Lath:
Can be used to create key on timber lintels, timber wall
plates, and is very useful to overcome situations involving
composite backgrounds.
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Prepare Backgrounds
Creating Key:
Natural Key:
Is the open texture found on some backgrounds such as
breeze block and some brickwork.
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Prepare Backgrounds
You should now have the knowledge to complete the following
What do you know about backgrounds ?
Try answer the following questions
1 What is suction
2 How can suction be controlled
3 How can suction be created
4 What is mechanical key
5 What is a bonding agent
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Private and Confidential
16