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Level 1 Diploma in Plastering
© 2013 City and Guilds of London Institute. All rights reserved.
PowerPoint
presentationCommon faults in plastering
Unit 121: Prepare background surfaces and mix
plastering materials

Assessment criteria
1.6 list common faults caused by ineffective surface
preparation
2.4 apply suction test and control measures on background
surfaces

Most faults in plastering are caused by poor
preparation of the background or the incorrect plaster
type being used.
In this PowerPoint presentation we will look at the
most common faults and the remedies.
Plastering faults

Fault: A very porous background sucking water
out of the backing plaster; this shows as small
cracks in the plaster.
Remedy: A simple test prior to starting plastering
is to throw water on to the wall. If the wall sucks in
the water, the wall will either need a slight soaking
or a bonding agent applied.
Drying out of backing plaster

Fault: Dust. Usually, after hacking off old
plaster, the wall is very dusty, which can
lead to bond failure and ultimately the
plaster falling off the wall.
Remedy: Brush the surface; dust off and
wash down with water.
Backing plaster bond failure can be
caused by:
• incorrect plaster type used for the
background, eg Browning grade on
dense smooth concrete
• insufficient key
• no bonding agent applied.
Backing plaster bond failure

Cause: Finish plaster shelling
off is caused by insufficient
key on the backing coat; the
finish plaster has nothing to
grip to and shells off, usually
in large sheets.
Remedy: when floating walls,
just before full set has taken
place rub the wall with a devil
float – a float with slightly
protruding nails which scratch
the surface, forming a key.
Shelling off backing coats

Plaster applied on existing
plaster or textured coatings is
only as good as the
background. If the wall has
peeling paint, this must be
completely removed prior to
applying a bonding agent; a
common example of bond
failure due to this is bubbles
appearing when trowelling up.
Shelling on re-skimmed walls

Causes:
• insufficient bonding agent, eg PVA being applied
• not applying the plaster when the PVA is tacky
• wrong dilution of PVA.
Remedies:
• apply more PVA
• make sure PVA is tacky when applying plaster
• use manufacturer’s recommended dilutions.
Shelling on re-skimmed walls

Cracking at wall plates and timber in the
background
This can be remedied by fixing EML over the wall
plates and timber in the background.
Cracking

Long, straight cracks or cracks
that follow brickwork patterns
could indicate a structural fault
in the building and no amount of
plastering will cover them, as
they will keep reappearing.
The best advice to give is for a
building surveyor to look at the
building.
Cracking

Prior to plastering, small cracks
must be covered by fibre tape
or paper tape to prevent cracks
reappearing through the newly
re-skimmed walls.
Cracking on re-skimmed walls

Crazing (or fire cracking,
as it is also known) is
caused by the backing
plaster drying out and
sucking the water out of
the finish plaster; this
can also be caused by
heat. Sometimes this
type of damage is seen
in fire-damaged houses.
Crazing

Remedies: On dried-out
backing plaster, soak the wall
with water or apply copious
amounts of PVA; this is the only
way to control the suction on
dried-out backing plaster.
Crazing

Causes: Bonding agents
not only act to bond the
plaster to surfaces but
they also seal surfaces
and control suction;
insufficient bonding
agents let suction take
place.
Remedies: Apply
sufficient bonding agent
as per manufacturers’
instructions.
Crazing on re-skimmed surfaces

Any questions?

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