PLASTER APPLICATION TOOLS AND TECHNIQUES

A plaster layer must have uniform thickness to attain complete structural integrity. Also, a plane plaster surface must be flat enough to appear flat to the eye and receive surface-applied materials, such as casings and other trim, without the appearance of noticeable spaces. Specified flatness tolerance is usually 1/8 inch in 10 feet.

TOOLS

Plastering requires the use of a number of tools, some specialized, including trowels, hawk, float, straight and feather edges, darby, scarifier, and plastering machines.

Trowels

Steel trowels are used to apply, spread, and smooth plaster. The shape and size of the trowel blade are determined by the purpose for which the tool is used and the manner of using it.

The four common types of plastering trowels are shown in figure 7-12. The rectangular trowel, with a blade approximately 4 1/2 inches wide by 11 inches long, serves as the principle conveyor and manipulator of plaster. The pointing trowel, 2 inches wide and about 10 inches long, is used in places where the rectangular trowel doesn’t fit. The margin trowel is a smaller trowel, similar to the pointing trowel, but with a square, rather than a pointed, end. The angle trowel is used for finishing comer angles formed by adjoining right-angle plaster surfaces.

Figure 7-12.—Plasterting trowels.

Hawk

The hawk (fig. 7-13) is a square, lightweight sheet-metal platform with a vertical central handle, used for carrying mortar from the mortar board to the place where it is to be applied. The plaster is then removed from the hawk with the trowel. The size of a hawk varies from a 10- to a 14-inch square. A hawk can be made in the field from many different available materials.

Figure 7-13.—Plasterting hawk.

Float

Afloat is glided over the surface of the plaster to fill voids and hollows, to level bumps left by previous operations, and to impart a texture to the surface. The most common types of float are shown in figure 7-14. The wood float has a wood blade 4 to 5 inches wide and about 10 inches long. The angle float has a stainless steel or aluminum blade. The sponge float is faced with foam rubber or plastic, intended to attain a certain surface texture.

Figure 7-14.—Plastering floats.

In addition to the floats just mentioned, other floats are also used in plasterwork. A carpet float is similar to a sponge float, but faced with a layer of carpet material. A cork float is faced with cork.

Straight and Feather Edges

The rod or straightedge consists of a wood or lightweight metal blade 6 inches wide and 4 to 8 feet long (see fig. 7-15). This is the first tool used in leveling and straightening applied plaster between the grounds. A wood rod has a slot for a handle cut near the center of the blade. A metal rod usually has a shaped handle running the length of the blade.

The featheredge (fig. 7-15) is similar to the rod except that the blade tapers to a sharp edge. It is used to cut in inside corners and to shape sharp, straight lines at outside comers where walls intersect.

Figure 7-15.-Straightedge and featheredge.

Darby

The darby (fig. 7-16) is, in effect, a float with an extra long (3 1/2 to 4 foot) blade, equipped with handles for two-handed manipulation. It is used for further straightening of the base coat, after rodding is 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.

Figure 7-16.-Darby.

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.

Figure 7-17.-Scarifier.

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.

Gypsum

The sequence of operations in three-coat gypsum plastering is as follows:

1. Install the plaster base.
2. Attach the grounds.
3. Apply the scratch coat approximately 3/16 inch thick.
4. Before the scratch coat sets, rake and cross rake.
5. Allow the scratch coat to set firm and hard.
6. Apply plaster screeds (if required).
7. Apply the brown coat to a depth of the screeds.
8. Using the screeds as guides, straighten the surface with a rod.
9. Fill in any hollows and rod again.
10. Level and compact the surface with a darby; then rake and cross rake to receive the finish coat.
11. Define angles sharply with an angle float and a featheredge. Trim back the plaster around the grounds so the finish coat can be applied flush with the grounds.

Lime

The steps for lime base coat work are similar to those for gypsum work except that, for lime, an additional floating is required the day after the brown coat is applied. This extra floating is required to increase the density of the slab and to fill in any cracks that may have developed because of shrinkage of the plaster. A wood float with one or two nails protruding 1/8 inch from the sole (called a devil’s float) is used for this purpose.

Portland Cement

Portland cement plaster is actually cement mortar. It is usually applied in three coats, the steps being the same as those described for gypsum plaster. Minimum recommended thicknesses are usually 3/8 inch for the scratch coat and brown coat, and 1/8 inch for the finish coat.

Portland cement plaster should be moist-cured, similar to concrete. The best procedure is fog-spray curing. The scratch coat and the brown coat should both be fog-sprayed cured for 48 hours. The finish coat should not be applied for at least 7 days after the brown coat. It too should be spray-cured for 48 hours.

FINISH COAT APPLICATION

Interior plaster can be finished by troweling, floating, or spraying. Troweling makes a smooth finish; floating or spraying makes a finish of a desired surface texture.

Smooth Finish

Finish plaster made of gypsum gauging plaster and lime putty (called white coat or putty coat) is the most widely used material for smooth finish coats. A putty coat is usually applied by a team of two or more persons. The steps are as follows:

1. One person applies plaster at the angles.
2. Another person follows immediately, straightening the angles with a rod or featheredge.
3. The remaining surface is covered with a skim coat of plaster. Pressure on the trowel must be sufficient to force the material into the rough surface of the base coat to ensure a good bond.
4. The surface is immediately doubled back to bring the finish coat to final thickness.
5. All angles are floated, with additional plaster added if required to fill hollows.
6. The remaining surface is floated, and all hollows filled. This operation is called drawing up. The hollows being filled are called cat faces.
7. The surface is allowed to draw for a few minutes. As the plaster begins to set, the surface-water glaze disappears and the surface becomes dull. At this point, troweling should begin. The plasterer holds the water brush in one hand and the trowel in the other, so troweling can be done immediately after water is brushed on.
8. Water is brushed on lightly, and the entire surface is rapidly troweled with enough pressure to compact the finish coat fully. The troweling operation is repeated until the plaster has set.

The sequence of steps for trowel finishes for other types of finish plasters is about the same. Gypsum-finish plaster requires less troweling than white-coat plaster. Regular Keene’s cement requires longer troweling, but quick-setting Keene’s cement requires less. Preliminary finishing of portland cement-sand is done with a wood float, after which the steel trowel is used. To avoid excessive drawing of fines to the surface, delay troweling of the portland cement-sand as long as possible. For the same reason, the surface must not be troweled too long.

The steps in float finishing are about the same as those described for trowel finishing except, of course, that the final finish is obtained with the float. A surface is usually floated twice: a rough floating with a wooden float first, then a final floating with a rubber or carpet float. With one hand the plasterer applies with the brush, while moving the float in the other hand in a circular motion immediately behind the brush.

Special Textures

Some special interior-finish textures are obtained by methods other than or in addition to floating. A few of these are listed beow.

STIPPLED.— After the finish coat has been applied, additional plaster is daubed over the surface with a stippling brush or roller.

SPONGE.— By pressing a sponge against the surface of the finish coat, you get a very soft, irregular texture.

DASH.— The dash texture is obtained by throwing plaster onto the surface from a brush. It produces a fairly coarse finish that can be modified by brushing the plaster with water before it sets.

TRAVERTINE.— The plaster is jabbed at random with a whisk broom, wire brush, or other tool that will form a dimpled surface. As the plaster begins to set, it is troweled intermittentl y to form a pattern of rough and smooth areas.

PEGGLE.— A rough finish, called peggle, is obtained by throwing small pebbles or crushed stone against a newly plastered surface. If necessary, a trowel is used to press the stones lightly into the plaster.