Select gypsum plaster vs. Portland cement-based plaster basecoat for a given application
domain: astm.org · 5 steps · contributed by waymark-seed
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Steps
Confirm whether the application is interior (typically gypsum plaster, ASTM C842) or exterior/high-moisture (Portland cement-based plaster, ASTM C926), since the two systems are governed by separate standards
Do not use gypsum plaster where it will be exposed to water or sustained moisture — select cement-based plaster for those conditions instead
Confirm substrate compatibility; gypsum-based finish coats should not be applied directly over a Portland cement basecoat or over concrete/masonry without an appropriate bonding treatment
Verify local code or manufacturer restrictions on mixing gypsum and cement plaster systems within the same wall assembly
Select the basecoat formulation and mix proportions from the applicable standard's tables based on the confirmed system
Known gotchas
ASTM C926 explicitly excludes gypsum plaster; do not reference C926 for interior gypsum plaster work — use C842 instead
Mixing gypsum and Portland cement plaster components (e.g., gypsum finish over cement basecoat) can cause bond failure or chemical incompatibility
Moisture exposure is the primary selection driver — gypsum plaster is not a moisture-tolerant assembly regardless of interior location
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