42 / 68
18
debonding and lack of hardness.
Crazing is usually the result of over
trowelling a rich (high cement con-
tent) mix, or the use of sand con-
taining an excessive amount of clay
material. Cracking and crazing can
occur due to the excessive loss of
water from plaster in the first hours
after application, exposure to the sun,
wind, or by suction into the walls.
Cracking can also develop after the
plaster has hardened as a result of
the use of poor quality sand, or the
application of thick layers.
Debonding is generally a result of
inadequate preparation of the sub-
strate, such as not cleaning dusty,
oily walls, or roughing up smooth
surfaces, and not allowing the wall
to reach the correct moisture con-
tent. It can also be caused by poor
technique and the incorrect use of
bonding agents.
Lack of hardness is associatedwith
insufficient cement in themix, the use
of sands contained excessive fines, a
mixwith poor water retention proper-
ties, the addition of extra water after
the first mixing or rapid drying out
which means incomplete hydration
of the cement.
GYPSUM-BASEDPLASTER
Gypsum based plaster must not be
mixed with a plaster made with com-
mon cement, as gypsum is a sulphate
compound, which attacks common
cement paste. This will cause swell-
ing, softening and disintegration of
the plaster.
CEMENT PLASTER
FINISHES
Smooth untreated cement plaster
rarely retains a satisfactory appear-
ance. It tends to become somewhat
patchy in appearance through slight,
unavoidable variations in surface
texture. Steel trowelling increases the
likelihood that the surface will craze
and should be avoided in external
applications. A wood float finish is
preferablewhen painting the surface.
PLASTERING
Before plastering commences, all
chases with the electrical, plumbing
conduits, boxes should be fixed into
position. The first coat of a three-coat
application is sometimes referred to
as a ‘scratch coat’ and is frequently
deeply scored to break the continu-
ous surface to enable the second coat
to be applied. Scoring comprises of
parallel lines about 20 mm apart and
5 mm deep. A plaster comb can be
used to score the plaster.
Plaster coats should ideally have a
lower strength than thematerial onto
which they are applied. Successive
coats should be thinner than the coat
to which it is being applied.
Plaster should not be applied
during extreme temperatures and
moisture conditions as it may affect
Continued
▶▶▶
Inspector checks brickwork and levels
A subsidy house nearing completion