2 COAT WORK SYSTEM
Two-Coat Work: A scratch coat is applied and allowed to take up (stiffen) slightly before the brown coat is applied with the same plaster mix. This constitutes one coat. The second coat would be the application of the finish plaster.
Any application of two-coat gypsum plaster over a solid substrate of concrete masonry. Masonry must be clean of any residual dirt, efflorescence or any other material that may compromise the bond. All masonry joints should remain unstruck, flush and smooth with the surface of the block. Depending on the condition of the block, it may be advisable to apply a bonding agent or in some cases attach metal lath to ensure the bond of the plaster.
Any application of two-coat gypsum plaster over a solid substrate of reinforced monolithic concrete. Concrete must be free of dirt, laitance, efflorescence and any form release or residual oil that may inhibit the bond. Small imperfections or depressions and voids should be filled with compatible cement mortar and allowed to dry before plaster operations. Depending on the condition of the concrete it may be advisable to abrade by mechanical or chemical means or treat the surface with an appropriate bonding agent prior to the application of plaster. In some cases it may be advisable to attach metal lath to ensure the bond of the plaster.
Advances
Over the years, manufacturers have developed plaster products with lightweight aggregates. These types of products are desirable from an application standpoint because they put much less strain on the plasterer in the application process. They are also beneficial for machine application where the plaster has to be pumped through hoses and have been reported to have some insulation value. The drawbacks are that they are often a little gummier in composition, which also makes them a little more difficult to spread and attain slightly less compressive strengths than sanded plaster. Additionally manufacturers have products that include wood fiber in the mix. Originally used to supplement plastering operations that did not have access to quality sand, wood fibers also reduce the weight of gypsum plaster by 25% and provide better fire resistance than sanded gypsum plasters. Some gypsum plasters have also been developed for higher strength for security wall systems, corridors, schools and other applications that may require better performance characteristics.
Experience has shown that base coat composition, texture desirability and performance all play a role in the selection of plaster finish coats:
• Gauging Plasters: When proportionally mixed with lime putty provide a controlled set, with early hardness that help prevent shrinkage cracks.
• Keenes Cement: Retemperable gauging plaster mixed with lime putty provides a sand float finish.
• Manufactured finishes: Provide the best attributes in terms of surface hardness, wear ability and abrasion resistance.
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