NASCLA Accredited Commercial General ContractorFinishesMedium
A contractor is installing gypsum plaster on a ceiling. To ensure proper bond and prevent cracking, what is the most important preparatory step for the substrate (e.g., lath)?
- AInstall metal corner beads and control joints.
- BApply a scratch coat with reinforcing mesh.
- CEnsure the substrate is clean, dust-free, and properly wetted.
- DApply a bonding agent evenly to the entire surface.
Show answer & explanationAnswer & explanation
Correct answer: C. Ensure the substrate is clean, dust-free, and properly wetted.
For gypsum plaster, ensuring the substrate is clean, dust-free, and properly wetted is crucial. A clean surface allows for proper mechanical and chemical bond, while wetting prevents the dry substrate from rapidly absorbing water from the plaster, which can lead to premature drying, poor hydration, and cracking.
Why the other options are wrong
- A. Corner beads and control joints are important for finish and crack control, but they are installed after or during plaster application, not primarily as a 'preparatory step for the substrate's bond'.
- B. Applying a scratch coat is the first layer of plaster, not a preparatory step for the substrate itself, and reinforcing mesh is used within the plaster layers.
- D. While bonding agents can be used, ensuring a clean and wetted surface is a more fundamental and universally critical preparatory step for proper plaster adhesion.
Plaster Substrate Preparation
The process of readying the surface (lath, masonry, concrete) to receive plaster, focusing on ensuring proper cleanliness, moisture content, and bond-enhancing conditions.
- Cleanliness removes bond inhibitors (dust, grease, loose material).
- Wetting controls the suction rate of the substrate, preventing rapid water loss from plaster.
- Proper preparation is critical for preventing delamination, cracking, and premature drying.
Memory trick: Clean, wet, and ready - for plaster's bond, steady.