Last reviewed 16 Sept 2026 · 10 min read
Plastering
Plastering is the application of a thin layer of mortar (plaster) over walls, ceilings and other surfaces.
Objects
- Protect masonry from rain, weathering and atmospheric agents.
- Provide a smooth, even, hygienic surface for paint, whitewash or other finishes.
- Conceal defective workmanship and inferior materials.
- Improve appearance (decorative textures).
- Seal joints and reduce dampness penetration; improve fire resistance and sound insulation to some extent.
Plaster mortars
| Mortar | Features / use |
|---|---|
| Cement plaster | Cement and sand, commonly 1 : 3 to 1 : 6; strong, durable, water resistant — external walls, damp areas, internal walls; richer mixes (1 : 3, 1 : 4) for ceilings and exposed surfaces |
| Lime plaster | Fat or hydraulic lime with sand/surkhi; good workability, flexible, breathable — heritage and interior work; slow hardening |
| Cement–lime (composite) plaster | Improved workability and water retention with cement strength; less cracking |
| Gypsum plaster | Ready-mix gypsum — smooth interior finish, quick setting, no water curing; not for external/wet areas |
| Mud plaster | Clay, sand, straw/cow dung — low-cost rural buildings; needs protective wash |
Special plasters and finishes
| Plaster | Description |
|---|---|
| Stucco plaster | Decorative plaster for external (and internal) surfaces, commonly applied in three coats (scratch, fine and finishing coats) — imitates stone |
| Sand-faced plaster | Final coat finished by sprinkling or rubbing sand to give a uniform sandy texture |
| Rough cast (spatter dash) | Mortar containing coarse aggregate thrown onto the wall — rough, weather resistant external finish |
| Pebble dash | Pebbles or crushed stone thrown into the fresh plaster and pressed in |
| Textured / scraped / combed finishes | Patterns formed in the final coat |
| Waterproof plaster | Cement plaster with waterproofing admixtures |
| Acoustic plaster | Porous plaster (e.g. with gypsum and lightweight aggregates) — sound absorption |
| Barium plaster | Contains barium sulphate — X-ray shielding walls in hospitals |
| Keene's cement | Hard, white, polished finish obtained from burnt gypsum with alum — fine interior finish |
| Parian and Martin's cements | Hard gypsum-based finishing plasters |
| Scagliola | Imitation marble finish made from gypsum/Keene's cement with glue and colouring |
| Granite/marble chips plaster, grit plaster | Exposed aggregate decorative finishes |
Thickness and number of coats
| Surface | Typical thickness |
|---|---|
| Internal brick walls | 12 mm (single coat) |
| External walls | 15–20 mm (often in two coats; e.g. 12–15 mm base coat + about 6 mm finishing coat) |
| Ceilings (RCC) | 6–10 mm |
| Rough stone/rubble masonry | Up to about 20–25 mm or more to fill irregularities |
- One-coat plaster — common for internal walls.
- Two-coat plaster — first (rendering) coat to fill irregularities, second (finishing) coat for a smooth surface — external walls.
- Three-coat plaster — rendering, floating and finishing coats — stucco and high-quality work.
Method of plastering
- Preparation of the surface
- Rake out mortar joints of masonry to a depth of about 10–12 mm (while mortar is green or by raking later) to provide a key.
- Remove dust, loose mortar, oil and efflorescence; roughen smooth concrete surfaces (hacking) or apply a bonding agent.
- Wet the surface thoroughly (so it does not absorb water from the plaster), but without standing water.
- Fix chicken wire mesh (galvanised welded mesh) across junctions of different materials (e.g. RCC columns/beams and brick masonry) and over service chases to prevent cracks.
- Fixing grounds (screeds/dots) — small patches (dots) of plaster at intervals, made plumb and level, joined into vertical strips (screeds) to act as thickness guides.
- Application — the base coat is thrown/dashed onto the wall with a trowel and pressed in; filled between screeds.
- Levelling — a straight edge (aluminium/wooden) is worked over the screeds to remove excess and level the surface; low spots filled.
- Floating — surface rubbed with a wooden float to compact and smooth.
- Finishing coat (where specified) — applied after the base coat has partially set; finished with a steel trowel (smooth) or other tool for texture.
- Curing — keep plaster moist for at least 7 days (cement plaster), starting after initial set; protect from hot sun and drying wind.
- Work sequence — plaster the ceiling first, then walls, from top downwards; avoid joints in the middle of a panel (end at corners or bands).
Finishing treatments
- Neeru (lime putty) finish — a thin coat of well-matured lime putty trowelled smooth over lime/cement plaster (traditional).
- Punning — thin coat of plaster of Paris applied over cement plaster for a very smooth white surface.
- Wall putty — white cement–polymer based putty applied in thin coats and sanded — smooth base for paints.
Defects in plastering
| Defect | Cause |
|---|---|
| Cracking and crazing | Rich mortar and excessive shrinkage, rapid drying (no curing), thermal movement, structural movement, junctions of different materials without mesh, thick single coats |
| Blistering | Expansion of unslaked lime particles in lime plaster; also moisture beneath finish coats |
| Popping | Small conical pits formed as particles of unslaked lime or other expansive matter expand and blow out |
| Efflorescence | Soluble salts in bricks, sand or water brought to the surface by moisture |
| Peeling / flaking / hollow (drummy) sound | Poor bond — dry or dusty background, smooth surface without key, joints not raked, plaster applied too thick or on a surface that absorbed its water |
| Softness / powdering | Poor-quality or insufficient binder, excess sand, inadequate curing, dampness |
| Rust stains | Corroding iron embedded in or behind plaster |
| Uneven surface | Poor workmanship, lack of screeds, improper floating |
| Dampness patches | Leakage, rising damp, porous plaster |
| Grinning | Joints of masonry visible through thin plaster due to differential suction |