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Straight and Feather Edges
Finish Coat Application

Builder 3&2 Volume 02 - Construction manual for building structures
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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







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