Plastering and Finishing Works
What this domain tests
Plastering questions target thicknesses, mortar mixes for different surfaces, preparation of the backing, curing, and the diagnosis of defects such as crazing, efflorescence and debonding. Finishing coats, putty and surface hardness also appear.
Core rules and concepts
- Single-coat plaster 12 mm thick; two-coat work 20 mm total (12 mm undercoat plus 8 mm finishing coat).
- Mixes: ceiling 1:3, external wall 1:4, internal wall 1:6, and 1:3 to 1:4 for backing coats on rough surfaces.
- Rake out masonry joints 10-15 mm and hack RCC surfaces for key; fix 150 mm wide chicken mesh over masonry-to-RCC junctions to prevent cracking.
- Wet the background before plastering, apply from bottom to top, cure for at least 7 days.
- Sand-faced or roughcast plaster is used externally to hide undulations; a 1.5-2 mm putty coat is applied for painting.
- Common defects: crazing from rich mixes, efflorescence from soluble salts, blistering from damp or trapped moisture, and popping from unslaked lime particles.
Worked example
Cement for 100 m² of 12 mm plaster in 1:6. Wet volume = 100 × 0.012 = 1.2 m³; dry volume = 1.2 × 1.30 = 1.56 m³. Cement = 1.56 ÷ 7 = 0.223 m³ = 0.223 ÷ 0.035 = 6.4 bags, roughly 320 kg. Sand = 1.56 × 6 ÷ 7 = 1.34 m³. Add about 5% wastage for droppings and edges.
Traps that cost marks
Forgetting the dry-volume factor; using 1:6 for ceilings where 1:3 is required; deducting small openings from plaster area when measuring; confusing crazing (fine hairline map cracks) with structural cracking.