This is a process of covering rough surfaces with a plastic material to obtain an even, smooth, regular, clean and durable surface. Plastering conceals defective workmanship and covers up unsound and cheap quality material. Often, plastering is required to provide a satisfactory base for decorating the surface by white washing, colour washing, distempering or painting. External plastering also termed as rendering is done with the object of improving the resistance of the surface to rain water penetration and other atmospheric influence.
The plastic material or plaster is made by working together a mixture of
building materials which may be cement, lime or clay, fine aggregates and
water. Certain additives are sometimes added to improve its adhesi veness,
durability and lusture. When cement is used as the binding material, the
plaster is termed as cement plaster and when lime is used as the binding
material it is called lime plaster. Sand normally forms the greatest proportion
of the constituents of a plaster. Sand
controls the shrinkage, porosity, strength and adhesive properties of plaster.
Sand use should be clean, sharp and free from deleterious matter. Depending
upon the availability of the materials, the choice of plaster for any
particular location, is governed by the rainfall, weather conditions and the
finish desired.
PREPARATION OF SURFACE FOR PLASTERING
The durability of plaster depends to a great extent upon its adhesion
with the back ground. The preparation of surface for plastering is therefor of
prime importance. All the projections which extend by more than 13 mm. from the
general surface of the masonry wall face are knocked off to obtain a uniform
surface and this also helps to reduce consumption of plaster. In order to
obtain a good key for the plaster with the wall surface, all the joints in the
masonry are raked out for a depth of at least 13 mm.
To ensure required thickness and a true surface, patches of plaster 15 cm
x 15 cm. are first of all applied horizontally and vertically at about 1.8 m.
apart over the entire surface. The surface of the patches of plaster serves as
gauges for maintaining even thickness of the plaster being applied. Mortar is
then applied on the wall between the screed with towel.
LIME PLASTER
Lime used in plastering may be fat lime or hydraulic lime. Fat limes make
best plaster as they yield good putty after slaking. Hydraulic lime on the
other hand, yields harder and stronger plaster, but it may contain some
unslaked particles which may slake slowly on absorbing moisture from atmosphere
and damage the plastering by forming blisters. As a precaution the hydraulic
lime, if used, should be ground dry with sand, left for about 2 or 3 weeks and
then reground before use.
Mortar for lime plaster is usually prepared by mixing sand and lime in
equal proportions. Cement is small quantity is sometimes added to the mixture
to improve its strength. Gugal and chopped hemp are sometimes added at the rate
of 4.5 kg. and 2.7 kg. respectively to every 2.83 cu.m. of mortar. This
treatment prevents the formation of cracks in plaster on drying.
APPLICATION OF LIME PLASTER
It is usually to provide 13 mm. and 16 mm thick plaster on plain side and
rough side of one brick thick walls respectively. In case of rubble masonry 19
mm thick plaster is recommended to account for the roughness or unevenness of
the surface. Plaster may be applied in one, two, or three coats on the prepared
wall between the screed with the help of towel.
During the process of finishing, a solution of lime putty is applied on
the surface to make the surface smooth and workable. The finished surface is
then cured for at least seven days.
In case it is desired to apply plaster in more than one coat, the first
coat, also known as rough coat, is applied on the surface in a layer of 6 to 10
mm in thickness. In the first coat the mortar is dashed against the surface to
serve as a key to hold the next coats.
The third or finishing coat is applied 5 days after the 2nd
coat. This coat does not exceed 3 mm in thickness and usually consists of
mortar made with cream of white or fat lime with fine white sand taken in the
ratio of 1:2. This is well rubbed with wooden float and then finished with a
towel to a smooth and even surface.
CEMENT PLASTER
Cement plaster forms an ideal coating for external renderings. It is
specially suited for damp conditions where non-absorbent wall surface is
essential. Cement plaster is usually applied in one coat. The thickness of coat
can be 12 mm, 15 mm or 20 mm depending upon the site conditions and type of
building. Sometimes, where the thickness of plaster is more than 15 mm or when
it is desired to have a finer finish, plaster is applied in two coats.
In case of plastering with one coat, cement plaster is applied on the
prepared surface between the screed with mason’s trowel. The surface is
levelled by means of wooden float and straight edges and finally polished with
a trowel.
In case plastering is required to be done in two coats, the first coat is
applied as described above with the only difference that the plastered surface
is not polished. The surface of the first coat when set is roughened with a
scratching tool to form a key for the 2nd or finishing coat. The finishing
coat is then applied over the first coat surface within 48 hours. This coat is
well trowelled and the surface finished smooth. The plaster should be kept wet
for at least 7 days.
MUD PLASTERING
This type of plastering is common seen in kuchha construction in villages
and in other structures of temporary character. This is the cheapest form of
plastering. Mud to be used for plastering should be made from earth free from
grass roots, gravel, stone grit etc. All the clods in the earth should be
broken and reduced to a fine powder. The earth is then mixed with plenty of
water in a pit, adding, chopped straw, hay or hemp at the rate of 33 kg/cubic
metre of earth. This mixture of earth and straw well flooded with water, is
kept for a least 6 days. During this period, the mixture is worked up at
intervals with foot o phawras so as to convert it into a homogeneous mass.
No curing is done in this case. Cracks formed during drying or portions
which sound hollow when tapped, are required by opening such patches out and
then treating the surface with a mixture of clay and cow dung.
STUCCO PLASTERING
Stucco is the name given to a decorative type of plaster which gives an
excellent finish. The surface plastered with stucco compares very much with marble
finish both in smoothness and elegance. Stucco plaster can be used for interior
as well as exterior surfaces. It is usually laid in three coats making the
total thickness of the plaster to about 25 mm. The first coat is called the
scratch coat; the second a finer coat, also known as the brown coat, and the
third coat is called the white coat or finishing coat.
Each coat should be permitted to dry thoroughly before the next is
applied.
Stucco for interior walls:
In this case the scratch coat consists of a coat of lime plaster 12 mm in
thickness. The second coat or the brown coat consists of lime plaster which is
made richer in lime and is 10 mm in thickness, and the finishing coat which is
3 mm thick consists of a mixture of finest lime and well ground white stone. The
finishing coat is carefully polished with a bag of linen containing moist chalk
in the first instance and then oil and chalk. The surface is finally rubbed
with oil only and finished smooth and bright.
Stucco for exterior wall:
In this case the mortar for the rough coat is prepared by mixing cement
and sand in the ration of 1:3 and then adding 10% by weight of hydrated lime to
the mixture. The thickness of the first coat in this case is also kept 12 mm.
The second coat has the same composition as the rough coat but it is 10 mm in
thickness. The finishing coat which may be 3 to 6 mm. in thickness consists of
a mixture of cement and sand taken in proportions of 1:2 to 1:2 ½ white or
coloured cement may used in finishing coat to obtain the desired shade.
PLASTER OF LATH
Lathing may be either of wood or that of expanded metal. Wooden lath is
used for forming a ground for plastering timber partition walls and ceilings of
timber floors. Wooden laths are liable to decay or get attacked by ants and
hence they are rarely used. Wooden lath consists of thin strips of well-seasoned
wood, about 25 mm in width and 90 to 120 cm. in length. Depending upon the
strain due to load of plaster, the thickness of the strips vary from 3 mm to 13
mm. the thicker strips are used in the ceilings where the strain is maximum
while the thinner ones are used for vertical sides of the wall. The strips are
nailed in parallel lines to the wooden frames. 10 mm gap is provided between
two successive strips for plaster. The strips are fixed so as to beak joints at
the butt ends. The plastering is then done in the usual manner.
Metal lathing is available in a wide range of designs. They may be
defined as 1.Expanded metal, 2. B.R.C. fabric, 3. Ribbed expanded with forming
an integral part of the sheets 4. Perforated 5. Dovetailed and so on. Plain or
expanded metal lathing is commonly used in residential buildings. The lath is
stretched tightly as much as possible and then fixed to the wooden supports by
G.I. straps. In case of concrete or masonry surface, wooden plugs have to
embedded for fixing the lath. The lath sheets must be over lapped not less than
25 mm at the sides and ends and they should be fastened by galvanised lacing
wire running through the meshes.
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