Module 2: Surface Preparation & Base Coat Application
Plastering is only as strong as the surface it adheres to, making proper preparation and base coat application critical for a long-lasting, crack-free finish. This module covers the step-by-step process of preparing different surfaces (brick, block, concrete, drywall) and applying the first coat (scratch coat or base coat) to create a solid foundation for subsequent layers.
1. Surface Preparation
Before plastering, the substrate must be clean, stable, and properly primed to ensure strong adhesion:
Cleaning & Inspection: Remove dust, dirt, grease, loose mortar, or old flaky plaster. Check for structural cracks or dampness that may require repair before plastering.
Dampening Surfaces: Brick, block, and concrete should be slightly damp (not soaking wet) to prevent rapid moisture absorption from the plaster, which can weaken the bond.
Applying Bonding Agents: Smooth or non-porous surfaces (e.g., concrete, painted walls) need a bonding agent like PVA glue, SBR slurry, or mechanical keying (scratched surface) to help the plaster stick.
Mesh/Reinforcement Installation: On weak joints or cracks, fiberglass mesh or metal lath is fixed to prevent future cracking.
2. Base Coat Application (Scratch Coat)
The base coat (often a sand-cement mix or gypsum undercoat) provides strength and a level surface for the finishing coat.
Mixing the Plaster: Follow manufacturer ratios (e.g., 1 part cement to 4 parts sand for cement plaster). Mix to a smooth, lump-free consistency—neither too stiff nor too runny.
Applying the First Layer: Using a trowel and hawk, press the plaster firmly onto the wall in upward strokes (6–10mm thickness). Ensure even coverage without gaps.
Scratching/Keying the Surface: Before the plaster fully sets, use a scarifier, comb scratcher, or notched trowel to create horizontal grooves. This helps the next layer bond securely.
Checking for Flatness: Use a straightedge or spirit level to identify high/low spots. Fill depressions and scrape off excess for a uniform surface.
Curing: Cement-based plasters require moist curing (light misting) for 2–3 days to prevent cracking. Gypsum plasters air-dry naturally.
Common Mistakes to Avoid
Skipping surface prep, leading to delamination.
Over-troweling the base coat, which weakens the structure.
Applying plaster on dry surfaces, causing premature drying and cracks.
This module ensures learners master the foundational skills for durable plastering. The next module typically covers skim coating and finishing techniques.
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