Materials, Mixing, and Application of Plaster
Chapter Introduction
This chapter covers all the knowledge required for the Red Seal exam concerning plastering materials, their preparation (mixing), and their application. You must master not only the physico-chemical properties of the materials, but also mixing ratios, setting times, environmental application conditions, and applicable Canadian standards. An error in proportioning or material identification can lead to major defects (cracking, efflorescence, delamination) and failure of the practical exam.
2. Classification of Plastering Materials
2.1 Gypsum Plaster (Plaster of Paris)
Gypsum plaster is the base material of the trade. It is obtained by calcination of natural gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) at approximately 150–180 °C, which produces a hemihydrate (CaSO₄·½H₂O). This reaction is reversible: adding water regenerates the dihydrate in the form of interlocking crystals, which gives plaster its mechanical strength.
Key properties:
Bulk density: 800–1,200 kg/m³ (dry plaster)
Compressive strength: 2–10 MPa depending on type and water/plaster ratio
Setting time: 10–40 minutes depending on temperature and additives
Drying shrinkage: 0.1–0.5% (low, but significant on large surfaces)
Types of plaster according to CAN/CSA-A82.27:
| Type | Designation | Primary Use |
|---|
| P | Ordinary gypsum plaster | Base coats and interior finish |
| R | Quick-setting gypsum plaster | Repairs and urgent work |
| S | Slow-setting gypsum plaster | Large surfaces, hot climates |
| X | High-strength gypsum plaster | High-traffic areas, architectural finishes |
2.2 Lime (Air Lime and Hydraulic Lime)
Lime is used as a supplementary or primary binder in traditional plasters.
Air lime (Ca(OH)₂): Hardens by carbonation (absorption of atmospheric CO₂). Slow setting time (weeks). Used for decorative finishes and restoration.
Hydraulic lime: Contains calcium silicates and aluminates. Hardens by hydration, faster than air lime. Used for exterior plasters.
Typical ratio for a lime plaster: 1 volume of lime to 3 volumes of sand (1:3). The addition of fibres (horsehair, polypropylene) improves cohesion.
2.3 Portland Cement
Portland cement (CSA A3001 standard) is used for exterior plasters and wet areas. It offers high mechanical strength but greater shrinkage than gypsum plaster.
Types of cement:
Type GU (general use): The most common
Type HE (high early strength): For rapid repairs
Type MS (sulfate-resistant): For aggressive environments
Typical mix for exterior plaster (stucco): 1 volume of cement, 3 volumes of sand, 0.5 volume of lime (for workability). Water/cement ratio: 0.4–0.5 by mass.
2.4 Aggregates (Sand)
Sand forms the skeleton of the plaster. Its gradation and cleanliness are critical.
Requirements according to CSA A23.1:
Gradation: 0.1–5 mm (fine to medium sand)
Organic matter content: < 3% (NaOH test)
Fines content (passing 80 µm): < 5%
Moisture: < 5% (affects water/binder ratio)
Recommended gradation table (standard sieves):
| Sieve (mm) | % Passing (min–max) |
|---|
| 5.0 | 95–100 |
| 2.5 | 80–100 |
| 1.25 | 50–90 |
| 0.63 | 25–60 |
| 0.315 | 10–30 |
| 0.16 | 2–10 |
2.5 Additives and Admixtures
Accelerators: Aluminum sulfate, alums. Reduce setting time (winter use).
Retarders: Citric acid, borax, animal glue. Extend setting time (summer use, large surfaces).
Plasticizers: Improve workability without adding water (reduces shrinkage).
Fibres: Polypropylene, glass, horsehair. Control shrinkage cracking.
Water repellents: Silicones, stearates. Reduce water absorption (exterior plasters).
Caution: The compatibility of additives with gypsum plaster must be verified. Some cement-based accelerators cause a dangerous flash set.
3. Mixing and Preparation
3.1 General Principles of Mixing
Mixing must produce a homogeneous paste, free of lumps, with a consistency suited to the application. The fundamental rules:
47.Always add the binder to the water (never the reverse) to avoid lumps.
48.Water temperature: 15–25 °C ideally. Cold water (< 10 °C) delays setting; hot water (> 30 °C) accelerates it dangerously.
49.Mixing time: 2–5 minutes with a mechanical mixer. Over-mixing can break the crystals and reduce strength.
50.Rest time (slaking): 2–5 minutes after initial mixing, then a brief remix (30 seconds) before application. This allows complete hydration of the particles.
3.2 Water/Binder Ratio (Consistency Factor)
The water/plaster ratio (by mass) is the most critical parameter.
| Application | Water/Plaster Ratio (mass) | Consistency |
|---|
| Scratch coat | 0.55–0.65 | Stiff paste |
| Brown coat | 0.65–0.75 | Plastic paste |
| Skim coat | 0.75–0.85 | Fluid paste |
| Moulding and cornices | 0.50–0.60 | Very stiff paste |
Rule of thumb: A water/plaster ratio of 0.7 gives a paste that spreads easily but has a strength reduced by approximately 30% compared to a ratio of 0.5. Each increase of 0.05 in the ratio reduces strength by approximately 10%.
3.3 Quantity Calculations
Basic formula for plaster volume:
V = A × t
Where:
V = volume of plaster (m³)
A = surface area to cover (m²)
t = plaster thickness (m)
Example: Surface area of 40 m², thickness of 15 mm (0.015 m):
V = 40 × 0.015 = 0.6 m³
Quantity of dry plaster: Dry plaster occupies approximately 60% of the paste volume (the rest is water). Therefore:
Mass of plaster = V × 1,000 kg/m³ × 0.6 = 0.6 × 600 = 360 kg
Quantity of water: With a water/plaster ratio of 0.7:
Water = 360 × 0.7 = 252 litres
Waste and loss allowance: Add 10–15% for losses (droppings, cleanup, thickness variations).
3.4 Mechanical vs. Manual Mixing
| Criterion | Mechanical Mixing | Manual Mixing |
|---|
| Homogeneity | Excellent | Variable |
| Time | 2–5 min | 10–15 min |
| Maximum volume | Unlimited | ~20 L per batch |
| Risk of over-mixing | High | Low |
| Recommended use | All professional work | Small repairs |
Mixing speed: 400–600 rpm for gypsum plaster. Above 800 rpm, there is a risk of air entrainment and crystal breakage.
3.5 Environmental Conditions
Ambient temperature: 10–30 °C (ideal 15–25 °C). Below 5 °C, setting stops; above 35 °C, water evaporates too quickly.
Substrate temperature: Must be above the dew point to prevent condensation.
Relative humidity: 40–70% ideally. RH > 80% slows drying; RH < 30% accelerates evaporation and causes cracking.
Ventilation: Moderate air circulation is beneficial, but direct drafts on a freshly plastered surface cause uneven drying.
4. Application of Plaster
4.1 Substrate Preparation
Requirements according to CAN/CSA-A82.30:
81.The substrate must be clean, dry, free of dust, grease, flaking paint, or any contaminating agent.
82.Smooth surfaces (concrete, concrete blocks) must be roughened (scarified) or receive a slurry coat (cement grout) to improve adhesion.
83.Highly absorbent substrates (brick, terra cotta blocks) must be dampened before application to prevent rapid suction of water from the mix.
84.Metal lath must be fastened with nails or screws spaced at a maximum of 150 mm, with a minimum overlap of 25 mm at joints.
85.Wood lath: spacing of 6–10 mm between laths to allow mechanical keying of the plaster.
4.2 Three-Coat Application (Traditional Method)
The three-coat method is the reference for interior and exterior plasters.
Coat 1: The Scratch Coat
Thickness: 10–12 mm
Consistency: stiff
Application: sprayed or troweled on, then scratched (scored) diagonally with a notched trowel or comb
Setting time: 24–48 hours before the next coat
Function: Ensure mechanical adhesion of subsequent coats
Coat 2: The Brown Coat
Thickness: 8–10 mm
Consistency: plastic
Application: after dampening the scratch coat
Straightening: with a straightedge (darby) and trowel, precise leveling
Setting time: 7 days minimum before finishing (complete drying)
Function: Provide thickness and flatness
Coat 3: The Finish Coat
Thickness: 2–3 mm
Consistency: fluid
Application: with a stainless steel trowel
Possible finishes: smooth, floated, sponged, textured
Drying time: 24–48 hours before painting
Total thickness table:
| Type of Plaster | Minimum Thickness | Maximum Thickness |
|---|
| Interior on lath | 15 mm | 20 mm |
| Interior on masonry | 10 mm | 15 mm |
| Exterior (stucco) | 20 mm | 25 mm |
| Spot repair | 3 mm | 30 mm (in multiple passes) |
4.3 One-Coat Application (Modern Method)
For gypsum wallboard, application is done in one or two passes:
111.First pass: Application of joint compound in the joints and over the screws.
112.Tape: Placement of joint tape (paper tape or mesh tape) in the fresh compound.
113.Second pass: Covering the tape with a 1–2 mm layer.
114.Third pass (optional): Final smoothing with a thin layer (0.5–1 mm) over the entire surface (skim coat).
Drying time between passes: 12–24 hours depending on humidity and temperature.
4.4 Specific Application Techniques
Mechanical spraying (gunite/plaster machine):
Advantages: Speed, uniformity, application on large surfaces
Parameters: Air pressure 0.4–0.7 MPa, nozzle diameter 6–12 mm, spraying distance 300–600 mm
Caution: The mix must be stiffer (water/plaster ratio reduced by 0.05–0.1)
Trowel application:
Trowel angle: 30–45° for application, 10–20° for smoothing
Pressure: Constant, without excess (risk of burning the plaster)
Movement: Circular or zigzag, never linear (visible marks)
Float finish:
Use a felt or sponge float
Circular movement after the plaster begins to set (30–60 minutes)
Produces a fine, uniform texture
4.5 Managing Setting Time
The setting time of gypsum plaster is influenced by:
Temperature: Each 10 °C increase reduces setting time by approximately 50%
Water/plaster ratio: More water = slower set
Additives: Accelerators or retarders
Agitation: Prolonged mixing accelerates setting
Signs of setting:
136.The paste begins to thicken (initial set)
137.The surface becomes dull (loss of gloss)
138.The paste resists finger pressure (final set)
139.The surface hardens completely (hardening)
Golden rule: Never rehydrate a plaster that has begun to set. This destroys the crystalline structure and produces a plaster with no strength.
5. Applicable Canadian Standards and Codes
5.1 CSA Standards (Canadian Standards Association)
| Standard | Title | Application |
|---|
| CAN/CSA-A82.27 | Gypsum and gypsum products | Gypsum plaster, joint compounds |
| CAN/CSA-A82.30 | Application of plaster and stucco | Application methods, thicknesses |
| CAN/CSA-A82.31 | Metal lath | Requirements for lath and accessories |
| CSA A3001 | Hydraulic cement | Portland cement, types and properties |
| CSA A23.1 | Concrete and constituent materials | Aggregates, sand, mixing water |
5.2 National Building Code (NBC)
The National Building Code of Canada (NBC) contains requirements relating to plasters:
Article 9.29.1.1: Interior plasters must be applied in accordance with CAN/CSA-A82.30.
Article 9.29.2.1: Metal lath must conform to CAN/CSA-A82.31.
Article 9.29.3.1: Exterior plasters (stucco) must have a minimum thickness of 20 mm in three coats.
Article 9.29.4.1: Plasters on concrete must be applied on a roughened surface or with a slurry coat.
5.3 Safety Requirements (Canada Labour Code)
Canada Occupational Health and Safety Regulations (SOR/86-304): Respiratory protection during mixing (gypsum dust, crystalline silica).
WHMIS (Workplace Hazardous Materials Information System): Safety Data Sheets (SDS) mandatory for all materials.
Exposure limit value (ELV) for crystalline silica: 0.025 mg/m³ (8-hour time-weighted average).
6. Common Defects and Remedies
6.1 Cracking
| Type of Crack | Probable Cause | Remedy |
|---|
| Shrinkage cracks (fine, spider-web) | Water/binder ratio too high, drying too fast | Reduce water, dampen substrate, protect from drafts |
| Structural cracks (wide, through-cracks) | Substrate movement, excessive thickness | Check substrate, apply in thinner coats, use lath |
| Star cracks | Stress concentration (window corners) | Reinforce with metal mesh at corners |
6.2 Delamination
Cause: Application on a dusty, damp, or frozen substrate; scratch coat too smooth; lack of scratching.
Prevention: Thorough cleaning, controlled dampening, systematic scratching of the first coat.
6.3 Efflorescence
Cause: Migration of soluble salts to the surface with evaporating water.
Appearance: White crystalline deposits on the surface.
Treatment: Dry brushing, washing with clean water after complete drying. Prevention: water repellents, adequate drainage.
6.4 Blistering
Cause: Application of the finish coat before the brown coat is completely dry; trapped air.
Prevention: Respect drying times (7 days minimum), apply the finish coat in a thin layer.
6.5 Plaster Burn (Dry-Out)
Cause: Highly absorbent substrate that draws water from the mix before crystallization.
Appearance: Powdery, non-adherent surface.
Prevention: Dampen the substrate before application, use a primer/sealer.
7. Practical Calculations for the Exam
7.1 Unit Conversions
1 inch = 25.4 mm
1 foot = 0.3048 m
1 cubic yard (yd³) = 0.7646 m³
1 imperial gallon = 4.546 L
1 bag of plaster (standard) = 30 kg (Canada) or 25 kg (some manufacturers)
7.2 Calculating Wall Surface Area
Surface area = length × height
Example: Wall of 4.5 m × 2.7 m = 12.15 m²
Deductions: Doors (0.9 m × 2.1 m = 1.89 m²), windows (1.2 m × 1.5 m = 1.8 m²)
Net surface area = 12.15 − 1.89 − 1.8 = 8.46 m²
7.3 Calculating Plaster Volume for a Coat
Example: Net surface area of 8.46 m², thickness of 15 mm (0.015 m)
Volume = 8.46 × 0.015 = 0.1269 m³
Mass of dry plaster: 0.1269 × 600 kg/m³ = 76.14 kg
Number of 30 kg bags: 76.14 / 30 = 2.54 → 3 bags (round up)
Volume of water: 76.14 × 0.7 = 53.3 L
7.4 Estimating Setting Time
Setting time (min) = Reference time × (1 + 0.05 × ΔT)
Where ΔT = (20 °C − actual temperature) in °C
Example: Reference time = 30 min at 20 °C, actual temperature = 15 °C
ΔT = 5 °C
Time = 30 × (1 + 0.05 × 5) = 30 × 1.25 = 37.5 minutes
8. Pitfalls to Avoid
200.Adding water to a plaster that is beginning to set: Destroys the crystalline structure. Discard immediately.
201.Confusing gypsum plaster and Portland cement: Gypsum plaster must never be used outdoors (not moisture-resistant). Cement must not be used over gypsum (destructive chemical reaction).
202.Forgetting to scratch the scratch coat: Without scratching, the brown coat cannot adhere mechanically.
203.Applying the finish coat too early: The brown coat must be completely dry (7 days). Premature finishing causes blistering and cracking.
204.Ignoring substrate temperature: A frozen or overheated substrate prevents adhesion. Always check the surface temperature.
205.Using a water/plaster ratio that is too high to ease application: Strength drops exponentially. Use plasticizers rather than extra water.
206.Neglecting respiratory protection: Gypsum and silica dust are hazardous. Wear at least an N95 mask when mixing.
207.Mixing for too long: Over-mixing accelerates setting and reduces strength. Respect 2–5 minutes maximum.
208.Not checking the expiry date of materials: Gypsum plaster stored too long (over 6 months) loses its reactivity.
209.Forgetting control joints: On large surfaces (> 15 m²), control joints are mandatory to prevent cracking.
9. Summary
Gypsum plaster is obtained by calcination of natural gypsum and hardens by hydration (exothermic reaction).
The three coats of plaster are: scratch coat (10–12 mm, scratched), brown coat (8–10 mm, straightened), finish coat (2–3 mm, smoothed).
The ideal water/plaster ratio is 0.6–0.7 by mass. Each increase of 0.05 reduces strength by approximately 10%.
The ideal application temperature is 15–25 °C. Below 5 °C, setting stops.
The drying time between the brown coat and the finish coat is 7 days minimum.
Key standards: CAN/CSA-A82.27 (materials), CAN/CSA-A82.30 (application), CAN/CSA-A82.31 (lath).
The National Building Code (NBC) requires a minimum thickness of 20 mm for exterior stucco in three coats.
Common defects (cracking, delamination, efflorescence) are mainly due to errors in proportioning, substrate preparation, or environmental conditions.
Quantity calculations must include a waste allowance of 10–15%.
Safety: mandatory respiratory protection (silica), WHMIS, ELV of 0.025 mg/m³ for crystalline silica.
10. Exam Tips
224.Memorize the ratios: water/plaster (0.6–0.7), cement/sand/lime (1:3:0.5), coat thicknesses (10/8/2 mm).
225.Understand the chemical reactions: Calcination and hydration of gypsum are frequent questions.
226.Master unit conversions: Calculation questions often use both imperial and metric units.
227.Know the standards by their full names: CAN/CSA-A82.30, not just "CSA".
228.Identify classic traps: Questions on mixing errors and environmental conditions are recurring.
229.Practice volume and quantity calculations: Surface area × thickness = volume; volume × density = mass; mass / bag size = number of bags.
230.Review setting and drying times: 24–48 h between coats, 7 days before finishing, 24–48 h before painting.
231.Pay attention to extreme conditions: Frost, heat, humidity — each condition has specific requirements.
End of Chapter 3. This content covers all the Red Seal learning objectives for the "Materials, Mixing, and Application of Plaster" domain. A thorough review of this chapter, accompanied by practice on calculations, is essential to pass the exam.
