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PWA-104 Plastering

The standards of ASTM C 926 for mix and application apply to PWA 104, with a few variations. Mix Design: any code approved mix design is compliant and is suitable for PWA 104. It is recommended that designers specify mixes from ASTM C-926 and allow the plastering contractor to select his design of choice. Local plasterers know which mixes perform best with local sands and the application crews will typically perform better with mixes they are more familiar with. Never allow a mix design that is not in ASTM C-926 or have a current Evaluation Report proving code approval and/or equivalence. Additives: Be cautious of additives or ingredients promising to deliver characteristics to good to be true. Verify compliance, check past job history and, if allowed, strictly follow manufacturer's directions. Scratch Coat: apply scratch (first) coat to completed lath over foam. It is recommended to use a slightly richer (cement rich) scratch coat. Typically this will be 2 ½ to 3 ½ parts sand to combined cement volume. The richer scratch coat provides a strong matrix interwoven with the lath. Do not apply to frozen or wet rigid insulation. Score the fresh cement to provide a good key for the brown coat. Nominal thickness should be 3/8 inch. The thickness of scratch coat is important only to insure coverage of the lath. This means reasonable coverage and 100% coverage of the lath is not required. Brown Coat: The brown (second) coat is applied after the scratch coat has set. The brown coat mix should be slightly higher in sand than scratch coat. Typically this is 3 ½ to 5 parts sand to combined cement volume. The extra sand will have less shrinkage and provides a basecoat easier to float. Apply the second (brown) coat to fill the complete base to even with the established grounds (trim accessories). Allow the excess mix moisture to evaporate (surface) and then hard float the brown to provide a "densified", compacted complete basecoat. Hard floating makes the plaster more water-resistant and crack resistant. The hard floating process does not required hard pressure, but does require a float made from either cork, wood, neoprene rubber, hard felt or plastic. Curing: time between scratch and brown coats is not critical (see double-back). Immediate or same day, moist-curing is critical when the weather is hot, dry and/or windy. It is strongly recommended to moist-cure the basecoats to prevent accelerated evaporation of the cement. The need, amount and timing for moist-curing is largely dependent on the weather conditions immediately after application, or exposure to the sun.

Alternates: Double-Back: The scratch and brown coats may be applied in a double-back method. This means scratch and brown coats applied the same day. Soft Floating: Hard floating the brown is not necessary if a lamina is applied over the set brown coat. Lamina: the lamina is a skim coat of polymer-modified cement applied over the set brown coat and an alkali resistant fiberglass mesh ( 4 to 6 oz) is troweled into the skim coat. The fiberglass mesh should be completely embedded into the polymer-modified cement coat. This lamina adds water-resistance and is highly crack resistant, negating the need to hard float the brown coat. Adding a lamina is not standard plastering practice and must be clearly stated in the construction documents if desired. Verify the polymer-modified skim coat and finish coat are compatible. Finish Coat: the finish coat may be cement or an acrylic finish coat. There are newer more exotic finish coats that may also be used, such as hybrids or Venetian plasters. Additional design considerations: Allow brown coat to set a minimum 7 days prior to applying finish, better results are anticipated if the brown coat can be allowed to set for 14 days or longer prior to applying the finish coat. A lamina eliminates the 14 day or longer recommendation. Damp curing of the finish coat is not recommended Apply finish coat is a continuous "wet edge" from architectural break to architectural break. Avoid smooth finish, unless a lamina is specified. (Sand Finish texture) Consider selecting a 16/20 (medium) sand ratio over a 20/30 (fine) sand for better and more consistent results. Foam Decorative Shapes: Foam shapes have become popular in conjunction with cement plaster assemblies. Cornices, window surrounds, quoins and decorative bands are used to add decoration to buildings. Foam shapes can also be added to PWA 104. It is recommended for best performance that the foam shape be adhesively applied to the basecoat (brown) of cement plaster and prior to the finish coat. The adhesive should be one manufactured expressly for the purpose of adhering foam to cement. The shape should be covered with a fiberglass mesh and polymer enriched cement expressly made to cover foam shapes. Insure

the cement and finish coats are compatible. It is most common to use EPS foam for shapes. These shapes do not negate the fire rating of a system if applied over the cement plaster for one hour walls and cement plaster and gypsum sheathing for two hour rated walls. Polyurethane coated shapes are not recommended. Alternate: Foam shapes of alternate materials may be used with approval from your local plaster bureau office.


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