Applying Plaster

Whether plaster is applied by hand or machine, it must be applied with sufficient force to develop full intimate contact between the plaster and substrate, and to put in place sufficient material to obtain the thickness in the coat. The applied plaster must be brought to the desired thickness, the walls made plumb, and the ceilings made level. The thickness of the coat of plaster is set by the use of screens and grounds. When the area between the screens and ground is filled with mortar, a rod is used to even the surface. The rod can bear on screeds or contact the grounds and be moved over the surface, cutting off high spots and showing up in the hollow places, which must be filled and rodded again. Additional manipulation of the surface is then required to prepare it for the next coat. Scratch coat plasters are scored / scratched to promote mechanical bond between it and the brown coat which is applied later. Brown coat plasters are applied and floated to even the surface and bring it to proper plane, to provide uniform suction throughout the basecoat plaster, and to provide a desirable surface for the finish coat. Brown coat surfaces are floated with a wood float to densify the plaster and to provide a surface that will improve bond with the final surface finish.

Both hand and machine application of plaster are common throughout the industry. Although procedures, tools, and equipment may differ, the method of application produces essentially the same plaster. For hand applied plaster, the water content is slightly less than for its machine applied counterpart. For machine applied plasters, the air content is slightly less than for its hand applied counterpart because entrained air within the plaster mixture is dissipated as the plaster is projected toward the surface by the pressurized air stream at the nozzle. Differences in the plaster associated with different application procedures are further reduced during rodding and floating. In place, plaster performance is assessed as equal for the two application procedures. However, machine applied plaster develops more intimate contact (bond) with a substrate.

Hand Application

The operation of hand applying plaster begins when the plaster for the scratch coat is placed on a mortarboard. The plasterer verifies that the plaster is properly mixed by kneading it on the board. The plasterer then removes the plaster from the board, puts it on the hawk, and begins plastering the base surface. For vertical surfaces, plastering can be done from bottom to the top of the work area, or from top to bottom. The direction depends on the type of work being done and local practice.

After transferring some of the plaster from the hawk onto the trowel, the plasterer lays the plaster on the surface. Enough pressure is applied to obtain good contact between plaster and base surface. This procedure continues until the entire surface is plastered to the desired thickness. Excessive troweling or movement of the scratch coat plaster must be avoided as it is being applied, because too much surface action will break the bond created between the plaster and the base, whether concrete, masonry, or metal lath.

The scratch coat should be uniformly scored in a horizontal direction on the vertical surfaces and in one direction on ceilings; shallow scratching is adequate.

The brown coat plaster is applied next in sufficient thickness, usually ¼ to 3/8 in. to bring the surface to the proper plane. A few minutes after the plaster has been applied, the surface is rodded to the desired plane with a darby. The plaster thickness is properly gauged with plaster screeds or wood slats of proper thickness as the guides. After rodding when the plaster achieves proper stiffness, the plasterer floats the surface to give it the correct surface texture.

Floating of the brown coat is the most important part of plastering. Floating must be done only after the plaster has lost sufficient moisture so that the surface sheen has disappeared and before the plaster has become so ridged that it cannot be moved under the float. This interval is critical, since the degree of consolidation that occurs during floating influences the shrinkage-cracking characteristics of the plaster.

The finish coat is applied to a predamped, but still absorptive base coat to a thickness of about 1/8 in. For vertical surfaces, the finish coat is applied from the top down and the whole surface must be covered without joinings, laps, or interruptions. The technique used to finish the surface of the finish coat depends on the surface texture desired.

Machine Application

Applying plaster by machine requires preplanning so the operation will go smoothly. The plaster pump should be placed adjacent to the mixer and have sufficient hose attached to the pump to allow quick and easy pumping of plaster from the mixer to the surfaces to be plastered. Hose lengths should be relatively straight and no longer than necessary. Normally, rigid pipe is used to accommodate the high pressures near the pump. Rigid pipe also offers less friction than a rubber hose. Reducing friction and subsequent pressure in the line helps reduce sand packing. Note that aluminum pipe or fittings should not be used to convey plaster. The pipe is coupled to a flexible rubber hose attached to the discharge nozzle. To ease the work effort demanded of the nozzle man, a light flexible whip line hose is used between the flexible rubber hose and the nozzle. The nozzle unit at the end of the pump contains valves that give the operator remote control of the pressurized-air and pump.

Before pumping plaster, the hose should be pre-wetted and lubricated to ensure unimpeded flow of plaster through the hose. Enough water is added to the pump holding tank to partially fill it. The pump is started and begins to move this water through the hose. The pump is stopped, the hose is disconnected at the pump, and a wet sponge is inserted in the hose. Safety considerations dictate that the whip line and nozzle not be connected to the hose at this time. The hose is reattached and the pump is started again. Water under pressure forces the web sponge through the hole. A neat cements slurry ( cement and water without sand) then is poured into the nearly drained pump holding tank. As pumping continues, the neat cement paste is forced through the hose, lubricating all the hose surfaces. When the sponge emerges from the hose at the discharge end, the nozzle man stops the pump, attaches the whip line and nozzle and sprays the remaining water and neat cement paste into a waste container. Since the neat cement paste is intended as lubrication for the hose, the operations involving line changes and nozzle additions should be completed as quickly as possible so the paste does not dry out or harden. The plaster mix is placed in the pump holding tank, pumped through the hose and detected at the nozzle by the presence of sand. When the mixture is uniform in appearance, it is applied to the surface to be plastered.

Good pumping practices require preconditioning, proper maintenance before and during pumping, and good cleanup and preventive maintenance of the hose upon completion of the work. Hose stoppages may occur during work due to poor mixtures or leakage at quick couplers located between pump and pipe, pipe and hose, hose and nozzle. A stoppage requires immediate removal of the obstruction and, if necessary, repair or replacement of equipment. Excessive line pressure will activate the pumps safety valve or possibly rupture a hose. Regular preventive maintenance of the pump should be an established procedure. When pumping is completed, the pump and hoses should be immediately cleaned. Water used to cleanse the mixer is discharged into the pump hopper, and the nozzle is removed from the hose. Water is then added to the hopper while it is cleansed. The fresh water is then pumped through the hose until the hopper is emptied. A sponge is inserted at the pump end of the hose, clean water is added to the pump hopper, and both sponge and water are pumped through the hose to complete the cleaning operation

During pumping applications, the nozzle man holds the nozzle approximately 12 in. from the surface. Plaster is applied to the desired, or slightly greater thickness. The nozzle man can vary the spray pattern and pattern size by adjusting the air pressure, changing the nozzle orifice size, varying the distance between the end of the air stem and orifice, increasing or decreasing pump speed, or calling for a change in water content to adjust the consistency of the plaster mix. Through proper selection of these options, the nozzle man can control the plaster application.

Machine application eliminates lap and joint marks, produces a more uniform appearance in color and texture, and produces, in color finish coats, deeper, darker, and more uniform colors then can be obtained by hand application.

For scratch and brown coats, the nozzle should be moved with a steady even stroke, laying on the proper thickness with one pass and overlapping successive strokes. The angle of the nozzle to the surface should be uniform. Around door bucks and window frames the nozzle should moved close, to within a few inches of the surface.

The same rodding, floating and finishing procedures used after hand application are used after machine application. Manufacturers of plastering machines publish instructions regarding their use, care and maintenance. These instructions should be carefully followed.