Why should the substrate be dampened before plastering?

Why should the substrate be dampened before plastering?

• A. To limit suction and improve adhesion
• B. To wash dust
• C. To color surface
• D. To speed up cure

Answer: (A)

Dampening the substrate is about controlling suction. Porous surfaces act like a sponge and pull water out of the plaster as soon as it’s applied. If the substrate is too dry, that rapid absorption can cause the plaster to lose moisture too quickly, leading to poor bonding, surface drying too fast, cracking, and a weakened initial set. Keeping the surface lightly damp reduces this suction, allowing the plaster to wet out evenly, bond firmly, and cure more uniformly. The surface should be damp but not pooling with water; too much moisture can dilute the plaster and weaken adhesion. Dampening isn’t about washing dust, changing color, or speeding up cure.

Dampening the substrate is about controlling suction. Porous surfaces act like a sponge and pull water out of the plaster as soon as it’s applied. If the substrate is too dry, that rapid absorption can cause the plaster to lose moisture too quickly, leading to poor bonding, surface drying too fast, cracking, and a weakened initial set. Keeping the surface lightly damp reduces this suction, allowing the plaster to wet out evenly, bond firmly, and cure more uniformly. The surface should be damp but not pooling with water; too much moisture can dilute the plaster and weaken adhesion. Dampening isn’t about washing dust, changing color, or speeding up cure.