Figure 7-17.-Scarifier.
completed, to level plaster screeds and to level finish
coats. The blade of the darby is held nearly flat against
the plaster surface, and in such a way that the line of the
edge makes an angle 45° with the line of direction of the
stroke.
When a plaster surface is being leveled, the leveling
tool must move over the plaster smoothly. If the surface
is too dry, lubrication must be provided by moistening.
In base coat operations, dash or brush on water with a
water-carrying brush called a browning brush. This is a
fine-bristled brush about 4 to 5 inches wide and 2 inches
thick, with bristles about 6 inches long. For finish coat
operations, a finishing brush with softer, more pliable
bristles is used.
Scarifier
The scarifier (fig. 7-17) is a raking tool that leaves
furrows approximately 1/8 inch deep, 1/8 inch wide, and
1/2 inch to 3/4 inch apart. The furrows are intended to
improve the bond between the scratch coat and the
brown coat.
Plastering Machines
There are two types of plastering machines: wet mix
and dry mix. The wet-mix pump type carries mixed
plaster from the mixing machine to a hose nozzle. The
dry-mix machine carries dry ingredients to a mixing
nozzle where water under pressure combines with the
mix and provides spraying force. Most plastering
machines are of the wet-mix pump variety.
A wet-mix pump may be of the worm-drive,
piston-pump, or hand-hopper type. In a worm-drive
machine, mixed plaster is fed into a hopper and forced
through the hose to the nozzle by the screw action of a
rotor and stator assembly in the neck of the machine. A
machine of this type has a hopper capacity of from 3 to
5 cubic feet and can deliver from 0.5 to 2 cubic feet of
plaster per minute. On a piston-pump machine, a
hydraulic, air-operated, or mechanically operated piston
supplies the force for moving the wet plaster. On a
hand-hopper machine, the dry ingredients are placed in
a hand-held hopper just above the nozzle. Hopper
capacity is usually around 1/10 cubic foot. These
machines are mainly used for applying finish plaster.
Machine application reduces the use of the hawk
and trowel in initial plaster application. However, the
use of straightening and finishing hand tools remains
about the same for machine-applied plaster.
CREWS
A typical plastering crew for hand application
consists of a crew leader, two to four plasterers, and two
to four tenders. The plasterers, under the crew leader’s
supervision, set all levels and lines and apply and finish
the plaster. The tenders mix the plaster, deliver it to the
plasterers, construct scaffolds, handle materials, and do
cleanup tasks.
For a machine application, a typical crew consists
of a nozzle operator who applies the material, two or
three plasterers leveling and finishing, and two to three
tenders.
BASE COAT APPLICATION
Lack of uniformity in the thickness of a plaster coat
detracts from the structural performance of the plaster,
and the thinner the coat, the smaller the permissible
variation from uniformity. Specifications usually
require that plaster be finished “true and even, within
1/8-inch tolerance in 10 feet, without waves, cracks, or
imperfections.” The standard of 1/8 inch appears to be
the closest practical tolerance to which a plasterer can
work by the methods commonly in use.
The importance of adhering to the recommended
minimum thickness for the plaster cannot be over-
stressed. A plaster wall becomes more rigid as thickness
over the minimum recommended increases. As a result,
the tendency to crack increases as thickness increases.
However, tests have shown that a reduction of thickness
from a recommended minimum of 1/2 inch to 3/8 inch,
with certain plasters, decreases resistance by as much as
60 percent, while reduction to 1/4 inch decreases it as
much as 82 percent.
7-14
