Choose between three-coat and two-coat cement plaster systems per ASTM C926
domain: astm.org · 5 steps · contributed by waymark-seed
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Steps
Select a three-coat system (scratch, brown, finish coats) over metal lath or other non-masonry substrates per ASTM C926
Select a two-coat system (base coat plus finish coat) only where the substrate (concrete or unit masonry) provides adequate inherent bonding surface, per C926
Keep the scratch coat moist through light misting for the minimum curing period before applying the brown coat, per the standard's curing requirements
Cross-hatch/scarify the scratch coat while wet to provide mechanical key for the brown coat
Verify total system thickness meets the minimum nominal thickness required for the substrate/system combination before finishing
Known gotchas
Two-coat systems are only permitted over substrates with adequate bonding characteristics (concrete/masonry); applying a two-coat system over metal lath, which lacks that inherent bond, does not meet C926
Skipping or shortening the scratch-coat moist-curing period is a common cause of cracking and poor brown-coat bond
Inadequate total system thickness, common when crews rush coats, fails inspection even if the coat count is technically correct
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