What is the consequence if the substrate is not sound?

• A. It has no impact on plaster performance.
• B. If substrate is not sound, bonding will fail, leading to cracks and delamination.
• C. It only affects the smell during application.
• D. Soundness is only a concern for exterior surfaces.

Answer: (B)

Substrate soundness directly governs how well plaster can grip and stay put. A solid, clean, stable base lets the bond coat adhere uniformly and develop a continuous interface. When the substrate is crumbly, loose, dusty, oily, damp, or cracked, the bond cannot form properly. This creates weak points where plaster can’t transfer stresses, so cracks form and the plaster can delaminate or peel away as movement or moisture occurs. That’s why the main consequence of an unsound substrate is bonding failure, leading to cracking and delamination. Proper substrate preparation—removing laitance, cleaning, dampening, patching, and using appropriate bonding methods—prevents these issues. The other options miss the real issue: substrate soundness matters for both interior and exterior plaster, and it’s about adhesion and crack/delamination risk, not odor or unrelated concerns.