Which factor must be within the recommended range to ensure proper bonding and curing of plaster?

• A. Surface temperature
• B. Ambient humidity
• C. Substrate moisture content
• D. Surface roughness

Answer: (C)

Substrate moisture content governs plaster bonding and curing because plaster needs an appropriate amount of water from the surface to hydrate and key to the substrate. When the substrate is within the recommended moisture range, it provides just enough suction to help the plaster grip without drawing water out too quickly. If the substrate is too dry, it pulls water from the plaster too fast, causing rapid shrinkage, surface cracks, and weak bonding. If the substrate is too wet, excess moisture can hinder set, reduce bond strength, or leave a weak, slow-curing surface. Keeping moisture within the specified range promotes proper hydration, good adhesion, and a solid cure. While surface temperature, ambient humidity, and surface roughness affect curing and finish, they don’t control bonding as directly as ensuring the substrate moisture is within the recommended range.