Interior and Exterior Plastering Systems

Introduction to Plastering: Roles and Application Contexts

Plastering is a surface finishing process that involves applying one or more coats of plaster to a substrate (masonry, concrete, metal lath, gypsum board, etc.) to achieve a smooth, durable, and aesthetically pleasing surface. In the context of the drywall finisher and plasterer trade, you must master both major families: interior systems (gypsum-based plaster) and exterior systems (cement-based finishes, often called stucco). The Red Seal exam requires you to understand material properties, application methods, environmental conditions, surface calculations, and national standard requirements.

This chapter covers fundamental principles, step-by-step procedures, mix and surface calculations, safety rules, and typical exam pitfalls. You must be able to distinguish between systems based on their use, substrate, and exposure to weather.

Types of Plaster and Their Properties

Gypsum-Based Plaster (Interior)

Gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) is calcined to produce the hemihydrate (CaSO₄·½H₂O). When mixed with water, the reverse reaction occurs: the plaster hardens through recrystallization. This reaction is exothermic and rapid. The setting time varies depending on the type of plaster and additives.

Type of Plaster	Primary Use	Typical Setting Time	Compressive Strength (MPa)
Gauging plaster	Interior finish coat	20–40 min	5–10
Brown coat plaster	Intermediate coat	60–90 min	8–12
Spray plaster	Large surfaces	30–60 min	6–10
Molding plaster	Cornices, ornaments	15–30 min	4–8

Golden rule: Gypsum plaster must never be used outdoors or in areas with constant humidity (commercial showers, pools) without specific protection. It is water-soluble.

Cement-Based Exterior Finishes

Exterior finishes are composed of Portland cement, lime, sand, and water. Portland cement is hydraulic: it hardens through a reaction with water, even underwater. The reference standards are CSA A3001 (Hydraulic cement) and CSA A82 for plasters. Typical proportions are expressed by volume.

Component	Typical Volume Proportion (3-coat)	Role
Portland cement	1 part	Hydraulic binder
Hydrated lime	0 to 1 part	Plasticity, water retention
Clean, graded sand	3 to 4 parts	Aggregate, mechanical strength
Clean water	Sufficient quantity for workability	Cement hydration

Important: Sand must be free of organic matter, silt, and salt. Sand that is too fine increases cracking; sand that is too coarse makes application difficult. The ideal particle size ranges between 0.15 mm and 4.75 mm.

Lime-Based Plaster

Hydrated lime (Ca(OH)₂) is used as an additive in cement finishes or as the primary binder in traditional plasters. It hardens slowly through carbonation (absorbing CO₂ from the air). Its use is limited today, but it remains relevant for heritage restoration. Standard CSA A82 covers quicklime and hydrated lime.

Interior Plastering Systems

Three-Coat Application (on Metal Lath)

The traditional interior system on metal lath comprises three coats:

19.Scratch coat: Base plaster applied to the lath, scratched horizontally to create mechanical adhesion. Thickness: 6 to 10 mm.

20.Brown coat: Applied after the first coat has partially set, it levels the surface. Thickness: 6 to 10 mm. The surface is straightened with a darby.

21.Finish coat: Smooth or textured finishing plaster, applied at 2 to 3 mm thickness.

Detailed procedure:

Verify that the lath is securely fastened (nails, screws, or staples) with at least 25 mm overlap between sheets.

Dampen the lath and substrate before application to prevent rapid absorption of water from the plaster.

Apply the first coat with pressure to force the plaster through the lath mesh.

Scratch the surface with a scratcher as soon as the plaster begins to set.

Allow to cure for 24 to 48 hours before the next coat.

Apply the brown coat, straighten it with a darby, and allow it to cure.

Apply the finish coat, ensuring the surface is damp but not saturated.

Application on Gypsum Board (Joints and Finishes)

Modern interior plastering often uses gypsum board with treated joints. The joint finishing system is classified into levels according to standard CAN/CSA A82.27 (Gypsum board application). Levels range from 0 to 5:

Level	Description	Typical Use
0	No finishing	Hidden areas
1	Joints covered with tape only	Non-visible ceilings
2	Tape + first coat of compound	Service areas
3	Tape + two coats of compound	Painted walls with diffuse lighting
4	Tape + three coats of compound	Painted walls with direct lighting
5	Complete smooth finish (fine sanding)	Raking light, critical surfaces

Red Seal rule: Level 5 is mandatory for surfaces subject to raking light (incidence angle less than 15°) or for high-gloss paints.

Spray Plastering

Spray-applied plaster (gunite or shotcrete for cement; sprayed gypsum for interior) is used for large surfaces. The machine projects the mix at high velocity, which compacts the material and improves adhesion. Critical parameters are:

Air pressure: 0.4 to 0.7 MPa (4 to 7 bars)

Spray distance: 300 to 600 mm from the surface

Spray angle: perpendicular to the surface

Water content: adjusted to avoid excessive rebound

Rebound (material that bounces off) must be less than 20%. Excessive rebound indicates a mix that is too dry or pressure that is too high.

Exterior Plastering Systems

Three-Coat Stucco

The traditional three-coat exterior system is the most common for residential and commercial buildings. It is applied over a rigid substrate (masonry, concrete) or over metal lath attached to a wood or steel frame.

Coats and thicknesses:

Coat	Minimum Thickness	Maximum Thickness	Function
Scratch coat	6 mm	10 mm	Adhesion to substrate
Brown coat	10 mm	20 mm	Leveling, strength
Finish coat	3 mm	6 mm	Aesthetics, relative waterproofing

CSA A82 standard requirements:

The substrate must be clean, stable, and dampened before application.

Metal lath must be galvanized and fastened with wide-head nails or screws (minimum 19 mm diameter).

Lath joints must overlap by at least 25 mm.

Each coat must cure completely before the next (minimum 48 hours at 20 °C and 50% relative humidity).

The finish coat must not be applied if the temperature is below 4 °C or above 35 °C.

One-Coat Stucco

The one-coat system uses a pre-mixed blend (cement, lime, sand, fibres, and additives) applied in a single coat of 10 to 20 mm. It is faster but requires a very uniform substrate. Standard CSA A82.4 (Application of Portland cement plaster) covers this system.

Advantages: speed, reduced cost, fewer risks of delamination between coats.

Disadvantages: less tolerant of substrate defects, potential cracking if the mix proportions are incorrect.

Exterior Insulation and Finish System (EIFS)

The EIFS system (standard CAN/CSA A118.1 for insulation boards) involves attaching expanded polystyrene (EPS) boards to the substrate, then applying a base coat reinforced with fibreglass mesh, followed by an acrylic finish coat.

Key points for the exam:

The mesh must be embedded in the base coat, with 65 mm overlap at joints.

A weather barrier is mandatory behind the EIFS.

Expansion joints must be provided every 6 metres maximum and at changes in plane.

EIFS is not a load-bearing system: it supports no structural load.

Surface and Material Calculations

Surface Area Calculation

The area to be plastered is calculated in square metres (m²). For a rectangular wall:

Area = Length × Height

For a ceiling:

Area = Length × Width

Example: A wall 6.5 m long and 2.8 m high has an area of 6.5 × 2.8 = 18.2 m².

Subtracting openings: Doors and windows are subtracted if their individual area exceeds 2 m². For a door of 0.9 m × 2.1 m = 1.89 m², you do not subtract. For a window of 1.5 m × 1.2 m = 1.8 m², you also do not subtract. However, a window of 2.0 m × 1.5 m = 3.0 m² is subtracted.

Plaster Volume Calculation

The volume of plaster required is:

Volume (m³) = Area (m²) × Thickness (m)

Example: For an area of 18.2 m² with a brown coat of 12 mm (0.012 m):

Volume = 18.2 × 0.012 = 0.2184 m³

Cement Mortar Mix Proportions

The volume mix for a standard cement finish is 1 part cement : 1 part lime : 4 parts sand (1:1:4). For a 40 kg bag of cement (approximately 33 L), the volume of mortar produced is:

Mortar volume = Cement volume + Lime volume + Sand volume + Water volume

With 20 L of mixing water (0.020 m³):

Volume = 0.033 + 0.033 + 0.132 + 0.020 = 0.218 m³

Rule of thumb: One 40 kg bag of cement produces approximately 0.20 m³ of 1:1:4 mortar. For 1 m³ of mortar, you need approximately 5 bags of cement.

Calculating the Number of Gypsum Plaster Bags

Gypsum plaster is sold in 30 kg bags. The typical yield is 0.015 m³ per bag for a standard mix (water/plaster = 0.7 by mass). For a 10 mm coat over 10 m²:

Volume = 10 × 0.010 = 0.10 m³

Number of bags = 0.10 / 0.015 = 6.67 bags → round up to 7 bags.

Exam trap: Always round up to the next whole number, because waste (5 to 10%) is not included in the theoretical yield.

Applicable Canadian Standards and Codes

National Building Code of Canada (NBC)

The National Building Code of Canada (NBC) 2020, published by the National Research Council (NRC), is the reference for minimum construction requirements. Relevant sections for plastering:

Section 9.28: Plaster and stucco — covers materials, application, and tolerances.

Article 9.28.1.1: Plaster must be applied in accordance with CSA A82.

Article 9.28.2.1: Substrates must conform to the requirements of Section 9.23 (framing) or 9.15 (masonry).

Key requirement: The total thickness of a three-coat exterior finish must not be less than 19 mm nor greater than 25 mm.

CSA A82 – Plaster and Stucco

Standard CSA A82 (Plaster and stucco materials) is the national standard for plastering materials. It specifies:

The chemical composition of gypsum and lime.

Purity requirements (minimum 66% gypsum for finishing plaster).

Test methods for compressive strength and setting.

CSA A3001 – Hydraulic Cement

Standard CSA A3001 (Cementitious materials for use in concrete) covers Portland cement types GU (general use), MS (moderate sulfate resistance), and HS (high sulfate resistance). For exterior finishes exposed to de-icing salts or sulfate soils, type MS is recommended.

CAN/CSA A82.27 – Gypsum Board Application

This standard defines joint finishing levels (0 to 5) and application methods. It is essential for interior finishing work.

Detailed Application Procedures

Substrate Preparation

Substrate preparation is the most critical phase. A poorly prepared substrate leads to inevitable delamination or cracking.

For masonry and concrete:

Clean the surface: remove dust, oils, efflorescence, and non-adherent material.

Dampen the surface before application to prevent rapid absorption of water from the plaster. The surface should be damp but without puddles.

For smooth concrete, apply a bonding agent or create mechanical roughness (scarification).

For metal lath:

Verify that the lath is taut and fastened at intervals of 150 mm maximum.

Edges must overlap by a minimum of 25 mm.

The lath must be at least 6 mm from the substrate to allow plaster anchorage.

Applying the Scratch Coat

115.Mix the plaster with clean water to a creamy consistency.

116.Apply with a trowel, exerting sufficient pressure to force the material through the lath mesh.

117.Level to a thickness of 6 to 10 mm.

118.Scratch the surface horizontally with a scratcher as soon as the plaster begins to set (test: your finger leaves a clean imprint).

119.Allow to cure for 24 to 48 hours.

Applying the Brown Coat

121.Verify that the scratch coat is hard and clean.

122.Dampen the surface lightly.

123.Apply the brown coat, straightening it with a darby to achieve a flat surface.

124.Final thickness: 10 to 12 mm.

125.Allow to cure completely (48 hours minimum).

Applying the Finish Coat

127.Mix the finishing plaster with water and possibly lime (for workability).

128.Apply in a thin coat (2 to 3 mm) with a stainless steel trowel.

129.Smooth the surface using circular or cross-hatch motions.

130.For a textured finish, use a sponge, brush, or special trowel before the plaster fully sets.

Temperature and Humidity Control

Gypsum plaster must not be applied if the ambient temperature is below 10 °C or above 35 °C. Relative humidity must be between 30% and 70%. For cement, the minimum temperature is 4 °C.

Consequence of temperature being too low: setting is delayed, final strength is reduced, and the risk of material freezing is high. Freezing of the mixing water causes irreversible disintegration.

Cracking: Causes and Prevention

Types of Cracks

Type	Primary Cause	Prevention
Shrinkage cracks	Drying too quickly, excess water	Control water content, moist curing
Settlement cracks	Unstable substrate, missing expansion joints	Provide joints, stabilize substrate
Spider-web cracks	Excess lime, sand too fine	Respect proportions, use graded sand
Delamination cracks	Coats applied too early or dirty substrate	Respect curing times, clean substrate

Expansion Joints

Expansion joints are mandatory in exterior finishes to accommodate thermal movement. The practical rule is to place them:

Every 6 metres maximum on continuous walls.

At changes in plane (corners, returns).

At junctions with windows and doors (100 mm from corners).

NBC requirement: Article 9.28.4.1 requires expansion joints in exterior finishes in accordance with CSA A82.

Occupational Health and Safety

Chemical Hazards

Quicklime (CaO): corrosive, causes chemical burns. Never add water to quicklime — always the reverse.

Portland cement: contains hexavalent chromium (Cr⁶⁺), a skin sensitizer. Use impermeable gloves and barrier cream.

Gypsum plaster: dust irritates the respiratory tract. Wear a minimum N95 mask.

Physical Hazards

Working at heights: use scaffolds conforming to CSA S269.2 (Scaffolds). Guardrails are mandatory above 1.2 m.

Manual handling: plaster bags weigh 30 to 40 kg. Use safe lifting techniques.

Noise: spray machines generate more than 85 dB(A). Wear hearing protection.

Personal Protective Equipment (PPE)

Safety glasses or face shield.

Nitrile or rubber gloves.

Steel-toe boots.

Hard hat on construction sites.

NIOSH-approved respirator for dust.

Pitfalls to Avoid

159.Confusing gypsum plaster and cement finish: gypsum is water-soluble, never for exterior use. Cement is hydraulic and can be used outdoors.

160.Forgetting to subtract openings: openings larger than 2 m² are subtracted from the surface calculation. Small openings are not.

161.Rounding down to the lower whole number: always round the number of bags up, with a 5 to 10% waste factor.

162.Applying the next coat too early: each coat must cure completely. Applying over a damp coat causes delamination.

163.Ignoring expansion joints: their absence causes inevitable cracking. The NBC requires them.

164.Using non-conforming sand: sand must be clean and graded. Fine sand increases shrinkage and cracking.

165.Neglecting curing: cement must be kept damp for 48 hours minimum after application for complete hydration.

166.Confusing finishing levels: level 5 is required for raking light, not level 3.

167.Forgetting the weather barrier: in EIFS systems, the moisture barrier is mandatory behind the insulation.

168.Working at inadequate temperatures: below 4 °C for cement, freezing destroys the material.

Summary

Interior plastering primarily uses gypsum (CaSO₄·½H₂O), which hardens through hydration. It is reserved for interior use.

Exterior plastering uses Portland cement (CSA A3001 standard), which is hydraulic, with typical proportions of 1:1:4 (cement:lime:sand).

Three-coat systems (scratch, brown, finish) are the standard for exterior finishes, with total thicknesses of 19 to 25 mm.

Surface calculations are done in m², subtracting openings larger than 2 m².

Material volume is Area × Thickness, with a 5 to 10% waste factor.

The NBC (Section 9.28) and CSA A82 are the mandatory references.

Expansion joints are required every 6 metres maximum on exterior finishes.

The minimum application temperature is 4 °C for cement and 10 °C for gypsum.

Safety requires complete PPE, including impermeable gloves for cement (hexavalent chromium).

Gypsum joint finishing levels (0 to 5) are defined by CAN/CSA A82.27.

Self-Assessment Questions

181.What is the fundamental difference between gypsum plaster and cement finish?

182.Calculate the number of gypsum plaster bags (30 kg, yield 0.015 m³/bag) for an area of 45 m² with a thickness of 12 mm.

183.What are the three coats of an exterior finish and their thicknesses?

184.At what maximum distance should expansion joints be placed in an exterior finish?

185.What finishing level is required for a surface subject to raking light?

186.What are the temperature and humidity conditions for applying gypsum plaster?

187.Why must sand be graded and clean in a cement finish?

188.What is the Canadian standard that covers plastering materials?

Answers: (1) Gypsum is water-soluble, cement is hydraulic. (2) Volume = 45 × 0.012 = 0.54 m³; bags = 0.54 / 0.015 = 36 bags, with waste → 40 bags. (3) Scratch 6–10 mm, brown 10–20 mm, finish 3–6 mm. (4) 6 metres. (5) Level 5. (6) 10 to 35 °C, humidity 30 to 70%. (7) Clean, graded sand reduces shrinkage and ensures uniform strength. (8) CSA A82.

Interior and Exterior Plastering Systems