Apply Finishes and Coatings

Applying Finishes and Coatings

Chapter Introduction

Applying finishes and coatings is the final and crucial step in manufacturing a piece of furniture or cabinetry. This operation goes beyond aesthetics: it protects the wood against moisture, scratches, stains, and dimensional changes. For the Red Seal exam, you must master the types of finishes, their properties, application methods, coverage calculations, and applicable safety standards. This chapter covers all the required knowledge, with an emphasis on common pitfalls and important distinctions between products.

Categories of Finishes and Their Characteristics

Solvent-Based Finishes (Lacquers, Varnishes, Shellac)

Solvent-based finishes are traditionally the most widely used in production cabinetry. They dry through solvent evaporation, which allows for rapid drying and an excellent level of gloss.

Nitrocellulose lacquer is the reference product for the exam. It is applied by spray gun, produces a hard but brittle film, and is easily repaired by dissolving with the original solvent. Note: it yellows slightly over time, which can be an advantage for aging a piece of furniture but a disadvantage for light woods.

Polyurethane varnish is a two-component finish (base + isocyanate hardener) for catalyzed versions. The mixing ratio is critical: excess hardener makes the film brittle, while insufficient hardener leaves it tacky. The minimum application temperature is 10°C, and relative humidity must be below 70% to prevent blushing.

Water-Based Finishes (Acrylics, Water-Based Polyurethanes)

Water-based finishes are progressively replacing solvents in the industry due to volatile organic compound (VOC) regulations. They dry through water evaporation followed by coalescence of resin particles.

The main challenge with water-based finishes is grain raising: water causes the wood fibres to swell, creating a rough surface after the first coat. The solution is systematic sanding between coats with 320 to 400 grit abrasive, or applying a grain filler before the finish coat.

Oils and Waxes

Oils (linseed oil, tung oil, Danish oil) penetrate the wood and polymerize in the air. They do not form a surface film, which gives a natural feel but limited protection. Boiled linseed oil dries in 24-48 h; tung oil in 12-24 h; Danish oil (a mixture of oil, varnish, and solvent) in 4-6 h.

Waxes (beeswax, carnauba wax) are applied over an existing finish or directly on the wood. Carnauba wax is the hardest (melting point 82-86°C) and most resistant; beeswax (melting point 62-65°C) is softer and easier to apply. Wax should never be applied to a fresh finish (less than 72 h old) because it would prevent residual solvent evaporation.

Surface Preparation

Sanding and Abrasives

Sanding is the most critical operation for final quality. The golden rule: each abrasive grit must remove the scratches from the previous grit. The standard progression is: 80 → 120 → 150 → 180 → 220 → 320 for an opaque finish, and up to 400-600 for a clear finish on fine woods.

Sanding between coats should be done with a grit equal to or finer than the initial preparation. Using a coarser grit risks cutting through the film and scratching the wood. For water-based finishes, always sand between each coat; for lacquers, light sanding at P400 is sufficient between the first two coats.

Dust Removal and Moisture Control

Dust removal is done with an antistatic cloth (tack cloth) immediately before application. Dust is the #1 cause of finish defects (pinholes, orange peel). Compressed air must be filtered and dry; excessive pressure (more than 2 bars) can embed dust into the pores.

Wood moisture content must be between 6% and 10% for interior cabinetry. Above 12%, the water contained in the wood can react with catalyzed finishes and cause blushing or delamination. Workshop relative humidity should be maintained between 40% and 60% during application and drying.

Grain Fillers and Sealers

Grain filler is a paste product applied to open-pore woods (oak, ash, walnut) to achieve a smooth surface before finishing. It is applied in the direction of the grain, excess is removed across the grain, then it dries 12-24 h before sanding at P320.

Sealer is an intermediate coat that seals the wood and prevents the finish from penetrating unevenly. For lacquers, shellac-based sealer is traditional; for water-based finishes, an acrylic sealer is recommended. The sealer must be sanded at P320 before applying the finish coat.

Application Methods

Spray Gun Application (Spraying)

Spraying is the dominant method in production. Three types of spray guns are used:

The HVLP spray gun is mandatory in most workshops to meet environmental standards. The spraying distance is 15-20 cm; too great a distance causes a powdery finish (dry spray), too short causes runs. The pass speed must be constant (approximately 30 cm/s) and passes must overlap by 50%.

Viscosity of the product is critical. It is measured with a viscosity cup (Ford cup #4): flow time should be 18-25 seconds for lacquer, 20-30 seconds for varnish, 15-20 seconds for water-based finish. Viscosity too high produces orange peel; too low produces runs.

Brush and Roller Application

Brush application is used for small surfaces, touch-ups, and oil finishes. The brush should be natural bristle for solvents (synthetic bristles dissolve) and polyester for water-based finishes. Application is done in three passes: one cross-grain pass, one with-the-grain pass, then a light levelling pass (laying off).

Roller application is reserved for large flat surfaces. A foam roller gives a smoother film than a fibre roller. To avoid bubbles, immerse the roller in the product then roll it on a grid before application.

Dip and Curtain Coating Application

Dipping involves immersing the part in a finish bath. This method is used for small parts (chairs, balusters) and produces a uniform film. Immersion time is 5-15 seconds, followed by draining for 10-30 minutes. Viscosity must be checked regularly because solvent evaporates from the bath.

Curtain coating is an industrial method where the part passes under a curtain of finish falling from a calibrated slot. Film thickness is controlled by line speed (20-60 m/min) and slot opening (0.1-0.5 mm). This method is reserved for flat surfaces in high-volume production.

Drying, Curing, and Cross-Linking

Drying Mechanisms

Evaporation drying (lacquers, shellac): the solvent evaporates, leaving the resin film. Drying time depends on temperature, humidity, and ventilation. A 10°C increase reduces drying time by approximately 50%.

Polymerization drying (oils, varnishes): resin molecules react with oxygen in the air (oxidation) to form a three-dimensional network. This process continues for several weeks; the film reaches its maximum hardness after 30 days.

Chemical reaction curing (catalyzed lacquers, polyurethanes): the hardener reacts with the resin to form a cross-linked polymer. The reaction is exothermic and depends on temperature. The pot life (usable time of the mixture) is 2-4 h at 20°C; beyond this, the mixture gels and must be discarded.

Drying Temperature and Humidity

Blushing is a defect caused by moisture condensation in the film during drying. It occurs when relative humidity exceeds 70% or when the part temperature is below the dew point. The remedy: warm the part and apply a retarder thinner that slows evaporation.

Finish Defects and Remedies

Common Defects and Causes

Defect Repair

Local repair of a minor defect (pinhole, small scratch) is done by localized sanding at P400, applying a thin coat of finish with a brush, then sanding at P600 and polishing. For more significant defects, you must strip the affected area with the appropriate solvent, then redo the complete finish.

Final polishing is done with a rubbing compound applied with a variable-speed polisher (1200-1800 RPM). The sequence: coarse compound → fine compound → polishing paste → protective wax. Polishing should never be done on a film with less than 7 days of drying.

Coverage and Quantity Calculations

Calculating the Surface Area to Cover

The surface area of a part to be finished is calculated by adding all exposed faces. For a panel with dimensions L × W × T, the total surface area is: 2 × (L × W) + 2 × (L × T) + 2 × (W × T). In practice, for a piece of furniture, a surface factor of 1.5 to 2.5 times the floor-projected area is used, depending on complexity.

Example: a wardrobe measuring 1.2 m × 0.6 m × 2.0 m (L × D × H). Floor-projected area: 1.2 × 0.6 = 0.72 m². Actual surface area to finish: 2 side panels (2 × 2.0 × 0.6 = 2.4 m²) + 1 back panel (1.2 × 2.0 = 2.4 m²) + 2 doors (2 × 1.2 × 2.0 = 4.8 m²) + 2 interior faces (2 × 1.2 × 0.6 = 1.44 m²) + shelves (3 × 1.2 × 0.6 = 2.16 m²) ≈ 13.2 m².

Product Coverage Rates

Coverage rate depends on wood porosity: oak absorbs 30-40% more product than maple. To calculate the required quantity: Quantity (L) = Surface Area (m²) × Number of Coats / Coverage Rate (m²/L) .

Example: surface area of 13.2 m², 3 coats of nitrocellulose lacquer at 10 m²/L. Quantity = 13.2 × 3 / 10 = 3.96 L. Add 10% for loss (spraying, cleaning): 4.36 L. Round up to 5 L.

Calculating Drying Time Between Coats

Drying time between coats depends on temperature and film thickness. The rule of thumb: drying time (min) = wet film thickness (µm) × temperature factor. The factor is 1.0 at 20°C, 0.5 at 30°C, 2.0 at 10°C.

Example: a lacquer coat of 100 µm at 20°C: 100 × 1.0 = 100 min. At 10°C: 100 × 2.0 = 200 min. At 30°C: 100 × 0.5 = 50 min. Note: drying too quickly (high temperature) can cause orange peel.

Canadian Standards and Regulations

Canadian Electrical Code, Part I, Chapter V

The Canadian Electrical Code, Part I, Chapter V (C22.1-21) governs electrical installations in areas where flammable liquids are used. For a spray booth, the following rules apply:

Grounding of the part being sprayed is essential: an electrostatic discharge can ignite solvent vapours. The part must be connected to ground with a copper wire of minimum 12 AWG, with a continuity resistance of less than 1 Ω.

CSA B149.1 — Natural Gas and Propane Code

CSA B149.1 applies to heating systems for spray booths. The main requirements:

The lower explosive limit (LEL) is the minimum concentration of vapours in air that can ignite. For toluene, the LEL is 1.1% by volume; for ethyl acetate, 2.0%. Ventilation must maintain the concentration below 25% of the LEL, which is 0.275% for toluene.

Composite Wood Products Regulations (SOR/2016-91)

This regulation, issued under the Canadian Environmental Protection Act (1999) , limits formaldehyde emissions from composite wood panels (MDF, particleboard, plywood). The limits are:

Finishes applied to these panels must comply with the requirements of CAN/ULC-S102 (flame spread test) if the furniture is intended for buildings subject to the National Building Code of Canada (NBC) .

Hazardous Products Act and WHMIS

The Workplace Hazardous Materials Information System (WHMIS) requires that all finishing products be accompanied by a Safety Data Sheet (SDS) . The mandatory elements: product identification, hazards, composition, first aid, fire-fighting measures, storage, exposure control, physical and chemical properties.

Occupational exposure limits (OELs) for common solvents:

Mandatory personal protective equipment (PPE) : cartridge respirator for organic vapours (Type A) for solvents, supplied-air respirator for isocyanates, nitrile gloves (solvent-resistant), safety glasses, and antistatic coveralls.

Quality Control and Inspection

Film Thickness Verification

Dry film thickness is measured with a film thickness gauge or a comb gauge. The recommended thickness for a quality finish:

Insufficient thickness reduces protection; excessive thickness causes cracking and a plastic appearance.

Adhesion and Resistance Tests

The cross-hatch test evaluates film adhesion: the film is scored into a grid of 6 × 6 squares of 1 mm, adhesive tape is applied, then pulled off. The result is rated from 0 (no removal) to 5 (complete removal). A result greater than 2 indicates an adhesion problem.

The solvent resistance test (MEK rub test) involves rubbing the film with a cloth saturated with MEK. A nitrocellulose lacquer film withstands 20-50 rubs; a catalyzed polyurethane film withstands 100-200 rubs. This test is destructive and must be performed on a test piece.

Inspection Conditions

Visual inspection must be done under lighting of 1000-1500 lux at an angle of 30-45° to the surface. The inspection distance is 30-50 cm. Acceptable defects depend on the finish class:

Safety and Environment

Product Storage and Handling

Finishing products must be stored in a ventilated room, away from ignition sources, at a temperature of 10-25°C. Containers must be tightly sealed and placed on spill containment trays. The maximum quantity stored is 250 L per room, in accordance with the National Fire Code of Canada (NFC) .

Oil-soaked rags (linseed oil, tung oil) present a spontaneous combustion risk: oil oxidation releases heat that can ignite the rag. They must be placed in a closed metal container filled with water, or laid flat to dry in open air.

Spray Booth Ventilation

Ventilation of a spray booth must provide an air exchange of minimum 0.5 m/s at the booth opening (face velocity). Airflow is calculated: Flow Rate (m³/h) = Velocity (m/s) × Opening Area (m²) × 3600.

Example: booth of 3 m × 3 m with an opening of 2 m × 2.5 m = 5 m². Flow rate = 0.5 × 5 × 3600 = 9000 m³/h. This flow rate must be provided by an exhaust system with particulate filtration (filters with minimum 95% efficiency) and, if necessary, a VOC treatment system (activated carbon, thermal oxidation).

Waste Management

Finishing waste (spray booth sludge, used solvents, containers) is hazardous waste under Canadian regulations. It must be stored in labelled, sealed containers and disposed of by an authorized carrier. Used solvents can be distilled for recovery; the distillation residue is hazardous waste.

Summary

Common Pitfalls to Avoid