Repair, Restore, and Refinish

Repairing, Restoring, and Refinishing

Introduction to the Repair and Restoration Module

Repair, restoration, and finishing work forms a significant part of the cabinetmaker trade. This chapter covers all the skills assessed by the Red Seal exam: damage assessment, selection of repair techniques, restoration of old finishes, and application of new finishes. You must master not only the technical procedures, but also the Canadian standards relating to materials, finishing products, and safety.

This module represents approximately 8 to 12% of the questions on the interprovincial exam. Questions focus on diagnosis, structural and aesthetic repair methods, finishing products, and dilution or coverage calculations.

Preliminary Assessment and Diagnosis

Furniture Inspection

Before any intervention, you must conduct a methodical inspection. This step determines the repair strategy and prevents further damage to the piece.

Inspection Procedure:

10.Visual examination under raking light (angle of 15 to 30°) to reveal warping, cracks, and areas of worn finish.

11.Palpation to detect areas of spongy wood, loose nails or screws, and the condition of the finish (sticky, crazed, flaking).

12.Joint integrity test: apply moderate pressure to frames, legs, and rails. Movement indicates a joint that needs to be re-glued.

13.Wood identification: species, grain direction, presence of veneer. This identification guides the choice of glues and stains.

14.Identification of the original finish: shellac, lacquer, varnish, oil, wax, or synthetic finish. A solubility test (see table below) is essential.

Table 10.1 — Finish Identification Test

Finish Type	Reaction to Denatured Alcohol	Reaction to Acetone	Reaction to Xylene
Shellac	Dissolves quickly	Dissolves	Resists
Nitrocellulose lacquer	Resists (slightly softens)	Dissolves quickly	Resists
Varnish (alkyd/urethane)	Resists	Resists	Softens slightly
Oil (boiled, tung)	Resists	Resists	Resists
Water-based finish (acrylic)	Resists	Resists	Resists

Rule of thumb: apply the solvent to a cotton swab and rub an inconspicuous area (under a leg, inside a drawer). If the swab picks up colour, the finish is soluble in that solvent.

Categories of Damage

Classify damage into three categories to determine the scope of work:

Surface damage: scratches, scuffs, cigarette burns, water stains, crazed finish. The wood structure is intact.

Structural damage: broken joints, cracks, split wood, lifted or missing veneer, missing parts.

Combined damage: deterioration of BOTH the finish AND the structure. Requires a sequential approach: structural repair first, then finishing.

Structural Repair

Glues and Adhesives

The choice of glue is critical. The following table summarizes the properties of glues commonly used in repair cabinetmaking.

Table 10.2 — Glues for Repair

Glue Type	Strength	Setting Time	Water Resistance	Typical Use
White glue (PVA)	Good	30–60 min	Low	Interior assembly, solid wood
Yellow glue (aliphatic)	Excellent	20–45 min	Medium	General joinery, assemblies
Hide glue (animal)	Good	Variable (gels at 18 °C)	Low	Antique restoration (reversible)
Urea-formaldehyde glue	Excellent	10–30 min	High	Veneering, marine plywood
Epoxy (two-part)	Excellent	5–60 min depending on formula	Very high	Repair of rotted wood, filling
Polyurethane glue	Excellent	30–120 min	Very high	Exterior wood, joints with gaps

Exam point: for restoring an antique piece of furniture, hide glue is preferred because it is reversible (it dissolves with heat and water). Restorers must be able to take apart an assembly without damaging the wood.

Clamping pressure calculation: the clamping pressure for a mortise-and-tenon joint must be sufficient to close the joint without squeezing out all the glue. Excessive pressure creates a "starved joint." The recommended pressure is 0.7 to 1.4 MPa (100 to 200 psi) for hardwoods, and 0.35 to 0.7 MPa for softwoods.

Repairing Checks and Splits

Check (drying crack): a longitudinal split caused by drying. If the crack is clean and without deformation, you can glue it by applying slight pressure with a clamp. Use wood glue and insert a thin veneer sliver or a hardwood wedge if the crack is wide.

Frost crack: a deep, irregular split. Requires doweling or invisible pinning:

34.Clean the crack and inject glue.

35.Insert hardwood dowels (oak, beech) 6 to 10 mm in diameter, spaced 75 to 100 mm apart.

36.Clamp and allow to dry for 24 hours.

37.Cut the dowels flush and sand.

Repairing a broken leg: for a turned or profiled leg broken in two, use a central dowel 8 to 12 mm in diameter. Drill an axial hole in each half, insert the dowel with epoxy, then clamp. The epoxy fills irregularities and ensures a strong bond.

Veneer Repair

Lifted veneer (bubbles) can be repaired by glue injection:

41.Slit the bubble with a scalpel along the grain of the wood.

42.Inject white glue or hide glue using a syringe.

43.Press with an iron (medium setting) or a veneer press.

44.Maintain pressure for 24 hours.

Missing veneer: cut a replacement piece from veneer of the same species and thickness (often 0.6 mm). Ensure the grain is aligned with the adjacent piece. Glue with hide glue or PVA glue, then sand with fine abrasive (grit 180 to 220).

Table 10.3 — Common Veneer Thicknesses

Veneer Type	Thickness (mm)	Use
Sliced veneer	0.6 – 0.8	Flat surfaces, panels
Sawn veneer	1.0 – 3.0	Edges, moldings, repairs
Reconstituted veneer	0.5 – 0.7	Decorative panels, large surfaces

Joint Repair

Mortise-and-Tenon Joints

A loose tenon can be repaired in several ways:

Simple re-gluing: if the tenon is intact and the mortise is not enlarged, clean, glue, and clamp.

Veneer shim: if the tenon has become thin, glue a thin veneer shim to one face of the tenon to restore the fit.

Reinforcement dowel: drill a through-hole through the tenon and mortise, insert a hardwood dowel with glue. This method is invisible if the dowel is placed in a hidden area.

Dovetail Joints

Loose dovetails are generally re-glued without complete disassembly. Inject glue into the joints, then clamp with bar clamps, protecting the edges with softwood cauls.

Caution: never force a dovetail joint with a metal mallet. Use a wooden or rubber mallet to avoid splitting the wood.

Metal Reinforcements

For furniture subject to high stress (chairs, tables), reinforcing brackets or mending plates can be added to the inside of the piece. These reinforcements should be discreet and fastened with wood screws of appropriate length (generally 25 to 38 mm).

Finish Restoration

Stripping Old Finishes

Stripping is necessary when the finish is too deteriorated to be repaired. Three main methods:

1. Chemical stripping (methylene chloride or solvent-based strippers)

Apply with a brush in a thick coat.

Let it sit for 15 to 30 minutes (the product lifts the finish).

Scrape with a plastic spatula (never metal on softwood).

Neutralize with denatured alcohol or water, depending on the product.

2. Thermal stripping (heat gun)

Heat the finish until it softens (temperature of 300 to 500 °C at the nozzle).

Scrape immediately with a spatula.

Risk: burning the wood, fire. Never use on finishes containing lead.

3. Mechanical sanding

Use an orbital sander with abrasives from grit 80 to 120.

Suitable for flat surfaces. For moldings, use flexible abrasives or abrasive brushes.

Safety rule: chemical stripping must be done in a ventilated area. Wear nitrile gloves (latex does not resist solvents) and a respirator with activated carbon cartridges.

Repairing Finishes Without Complete Stripping

Scuff-sanding: light sanding of the surface with a grit 220 to 320 abrasive to dull the existing finish before applying a new coat. This technique is used for touch-ups.

Scratch touch-up:

Surface scratch (in the finish only): fill with touch-up wax (wax stick) or a touch-up crayon, then buff.

Deep scratch (down to the wood): apply a touch-up stain, then a spray varnish or a French polish varnish.

Cigarette burn repair:

83.Scrape the burned area with a scalpel or razor blade.

84.Sand with grit 220 then 320.

85.Apply a touch-up stain (pigmented shellac).

86.Re-varnish locally.

Wood Bleaching

To remove water stains, mildew, or black rings (tannin oxidation), use chemical bleaching:

Oxalic acid (10% solution): effective on water stains and black rings. Apply, let sit for 15 minutes, then neutralize with a borax solution (4%).

Hydrogen peroxide + ammonia: for stubborn dye stains. Mix 1 part peroxide (30%) with 1 part ammonia (28%). Apply, let sit, then neutralize.

Table 10.4 — Bleaching Agents and Uses

Agent	Concentration	Use	Neutralization
Oxalic acid	10% in water	Water stains, black rings	Borax 4%
Hydrogen peroxide	30%	Dye stains, mildew	Clear water
Sodium hypochlorite (bleach)	5%	Ink stains, colorants	Clear water + vinegar

Caution: bleaching lightens the wood. Always test on a hidden area. Bleaching should not be used on oily woods (teak, rosewood) as it can create irregular blotching.

Applying Finishes

Choosing a Finish

The choice of finish depends on the use of the piece, the wood, and the original finish.

Table 10.5 — Finish Comparison

Finish	Durability	Water Resistance	Ease of Application	Repairability	Appearance
Shellac	Low	Low	Excellent	Excellent (soluble in alcohol)	Glossy, warm
Nitrocellulose lacquer	Medium	Medium	Good	Good	Glossy, hard
Polyurethane varnish	Excellent	Excellent	Medium	Low	Very glossy, plastic-like
Oil-based varnish (alkyd)	Good	Good	Good	Medium	Satin, warm
Oil (tung, Danish)	Medium	Medium	Excellent	Excellent	Matte, natural
Water-based finish (acrylic)	Excellent	Excellent	Good	Low	Clear, neutral

Golden rule in restoration: for an antique piece, use the same type of finish as the original. If the original finish is shellac, do not apply polyurethane varnish over it — adhesion will be poor and future repair will be impossible.

Surface Preparation

The quality of the finish depends 90% on surface preparation.

Sanding sequence:

104.Grit 120 to remove major defects.

105.Grit 180 to flatten.

106.Grit 220 for standard finishing.

107.Grit 320 for glossy finishes (lacquer, varnish).

Sanding between coats: use grit 320 or 400, or 0000 steel wool. Wipe off dust with a tack cloth.

Pore filling: for open-pore woods (oak, ash, walnut), use a wood filler (grain filler) before staining or finishing. The filler can be water-based or solvent-based. Apply with a brush, let dry, then sand with grit 320.

Staining

Stain colours the wood without masking the grain. Two main types:

Water-based stain: penetrates deeply, raises the grain (requires pre-wetting). Stable colours.

Alcohol-based stain (shellac-based stain): dries very quickly, ideal for touch-ups. Difficult to apply evenly over large surfaces.

Oil-based stain: penetrates slowly, easy to apply, rich colours. Requires a longer drying time.

Dilution calculation: for a water-based stain, the typical dilution is 1 part concentrated stain to 3 parts water (1:3 ratio). For an oil-based stain, the ratio is often 1:1 with paint thinner.

Colour test: always test the stain on a scrap of the same wood, under the same conditions. The final colour depends on the species, the grain, and the number of coats.

Applying Shellac

Shellac is a traditional finish made from lac resin dissolved in denatured alcohol. It is applied with a pad (French polish) or a brush.

Shellac preparation:

2-pound cut: 2 pounds of shellac flakes dissolved in 1 gallon of alcohol. In metric units, this corresponds to approximately 240 g of flakes per litre of alcohol.

3-pound cut: 360 g/L — used for base coats.

1-pound cut: 120 g/L — used for final coats and polishing.

Calculation formula: to prepare a 2-pound cut with 500 mL of alcohol:

Mass of flakes (g) = 240 g/L × 0.5 L = 120 g

French polish application:

126.Prepare a cotton pad wrapped in a fine cloth.

127.Saturate the pad with shellac (1 to 2-pound cut).

128.Apply in circular or figure-eight motions.

129.Allow 15 minutes drying time between coats.

130.After 3 to 5 coats, buff with a slightly oiled pad.

Working temperature: shellac must be applied at a room temperature of 18 to 24 °C. Below 15 °C, it becomes tacky and does not dry properly.

Applying Lacquer

Nitrocellulose lacquer is applied by spray gun. The viscosity must be adjusted according to the gun and temperature.

Table 10.6 — Lacquer Spraying Parameters

Parameter	Recommended Value
Air pressure	2.5 to 3.5 bars (250–350 kPa)
Gun nozzle	1.3 to 1.8 mm
Spray distance	15 to 25 cm
Dilution	10 to 20% lacquer thinner
Room temperature	20 to 25 °C
Drying time between coats	10 to 15 minutes

Dilution calculation: for 1 litre of lacquer to be diluted at 15%:

Volume of thinner = 1 L × 0.15 = 0.15 L (150 mL)

Applying Varnish

Varnish is applied by brush or spray. For brush application:

Use a natural bristle brush (hog bristle) 50 to 75 mm wide.

Apply in thin, even coats.

Between each coat, sand lightly with grit 320.

Polyurethane varnish requires a drying time of 4 to 6 hours between coats.

Table 10.7 — Finish Drying Times

Finish	Touch Dry Time	Recoat Time	Full Cure Time
Shellac (2 lb cut)	5–10 min	15–30 min	24 h
Nitrocellulose lacquer	5–10 min	10–15 min	24–48 h
Alkyd varnish	4–6 h	24 h	7 days
Polyurethane varnish	4–6 h	24 h	7 days
Danish oil	4–8 h	24 h	3–5 days
Water-based finish	30–60 min	2–4 h	7 days

Repairing Finishes After Application

Finish Defects and Corrections

Table 10.8 — Common Defects and Corrections

Defect	Cause	Correction
Orange peel	Viscosity too high, pressure too low	Sand, dilute further, reapply
Runs (sagging)	Coat too thick	Sand with grit 320, reapply in a thin coat
Bubbles	Trapped air, excessive agitation	Sand, reapply slowly
Blushing (white haze)	High humidity, drying too fast	Warm the room, apply a slow thinner
Poor adhesion	Greasy surface, insufficient sanding	Strip, clean, sand, reapply
Crazing (alligatoring)	Incompatible coats, finish too thick	Strip completely, start over

Treating blushing: blushing occurs when relative humidity exceeds 60% during lacquer application. To correct it, spray a slow thinner (retarder) on the surface, or warm the piece to 25 °C and reapply a thin coat.

Polishing and Buffing

The final polish gives the piece its professional appearance.

Polishing sequence:

155.Wet sanding with grit 400, then 600 abrasive (with soapy water).

156.Apply polishing compound with a 0000 steel wool pad.

157.Final buffing with a soft cloth and wax (beeswax or paste wax).

Rule: never polish an oil finish or a matte finish with abrasives. Polishing is reserved for glossy finishes only (lacquer, varnish, shellac).

Standards and Safety

Applicable Canadian Standards

Finishing products and solvents are governed by the Hazardous Products Act and the Hazardous Products Regulations (WHMIS) . The Workplace Hazardous Materials Information System (WHMIS) requires that all chemical products be accompanied by a Safety Data Sheet (SDS) .

Solvent-based finishes contain volatile organic compounds (VOCs) . Occupational exposure limits are established by the Canada Labour Code and provincial regulations (which vary — but the Red Seal exam focuses on general principles).

Essential safety rules:

Ventilation: minimum airflow of 0.5 m³/min per m² of surface in the spray booth.

Respiratory protection: respirator with activated carbon cartridges for solvents, particulate respirator (N95) for sanding.

Storage: solvents and finishes must be stored in fireproof cabinets, away from ignition sources.

Disposal: rags saturated with oil (linseed oil, Danish oil) can spontaneously combust. Place them in a closed metal container filled with water.

Surface Area and Coverage Calculations

Calculating the surface area of a panel:

Surface area (m²) = Length (m) × Width (m)

Example: a tabletop measuring 1.8 m × 0.9 m has a surface area of 1.8 × 0.9 = 1.62 m².

Calculating the amount of finish:

Quantity (L) = Surface area (m²) × Number of coats ÷ Coverage (m²/L)

Table 10.9 — Typical Finish Coverage

Product	Coverage (m²/L per coat)
Shellac (2 lb cut)	8–10
Nitrocellulose lacquer	10–12
Varnish	10–14
Oil	12–16
Water-based finish	8–10

Calculation example: to apply 3 coats of varnish to a surface of 2.5 m² with a coverage of 12 m²/L:

Quantity = 2.5 × 3 ÷ 12 = 0.625 L (round up to 0.75 L to account for losses).

Rule of thumb: add 10 to 15% extra product for losses (brush, gun, evaporation).

Pitfalls to Avoid

182.Not identifying the finish before stripping. Applying a stripper to shellac can partially dissolve it and create a sticky mess. Always test first.

183.Using PVA glue on an antique piece. PVA glue is not reversible. For antique restoration, use hide glue.

184.Sanding through the veneer. Veneer is 0.6 to 0.8 mm thick. Excessive sanding with grit 80 can cut through it. Use fine grits (180+) and sand by hand on edges.

185.Applying water-based stain without pre-wetting the wood. Wood absorbs water unevenly, creating blotches. Always wet the surface, let it dry, then sand lightly before staining.

186.Mixing incompatible finishes. Polyurethane varnish applied over shellac will peel. Respect chemical compatibility.

187.Forgetting ventilation when stripping. Methylene chloride vapors are toxic and heavier than air. Work in a ventilated area, ideally with source extraction.

188.Clamping a glued joint too tightly. Excessive pressure squeezes out the glue and creates a starved joint. Clamp until glue begins to squeeze out, then back off slightly.

189.Neglecting the solubility test. The Red Seal exam frequently tests the ability to identify a finish by its reaction to solvents. Memorize Table 10.1.

190.Calculating dilution incorrectly. Always re-read the manufacturer's instructions. A 15% dilution means 15 mL of thinner per 100 mL of product (or 150 mL per 1 L).

191.Applying finish in humid weather. Relative humidity above 60% causes blushing on lacquer. Check the hygrometer before spraying.

Summary

Diagnosis precedes any repair. Identify the wood, the finish, and the extent of damage before choosing a method.

Hide glue is the standard choice for antique restoration because it is reversible. PVA and epoxy glues are reserved for modern repairs.

The solubility test (alcohol, acetone, xylene) is the basic tool for identifying an unknown finish.

Surface preparation is essential: progressive sanding (120 → 220 → 320), dust removal, and pore filling for open-grain woods.

Finishes are chosen according to use and compatibility. Shellac is traditional and repairable; lacquer provides a hard, fast finish; polyurethane varnish is durable but difficult to repair.

Dilution and coverage calculations are frequent exam questions. Master the formulas: dilution (mL of thinner = volume × percentage) and quantity (surface area × coats ÷ coverage).

Safety is non-negotiable: ventilation, respiratory protection, storage of oil-soaked rags, and reading SDS sheets.

Canadian standards (WHMIS, Canada Labour Code) govern the use of chemical products. The exam tests your knowledge of general principles, not provincial regulations.

Self-Assessment Questions

204.A finish dissolves in denatured alcohol but resists acetone. What is this finish?

205.Which glue do you use to repair a drawer from a Victorian dresser? Justify your answer.

206.Calculate the amount of lacquer needed to apply 2 coats to a cabinet door measuring 0.6 m × 1.2 m (coverage 11 m²/L, 10% losses).

207.What is the recommended final sanding grit before applying a glossy varnish?

208.At what maximum relative humidity can lacquer be sprayed without risk of blushing?

209.What is the difference between a 1-pound cut and a 2-pound cut of shellac?

Answers: 1) Shellac. 2) Hide glue — reversible, compatible with antique furniture. 3) Surface area = 0.72 m²; quantity = 0.72 × 2 ÷ 11 = 0.131 L + 10% = 0.144 L (round up to 150 mL). 4) Grit 320. 5) 60%. 6) A 2-pound cut contains 240 g of flakes per litre of alcohol; a 1-pound cut contains 120 g/L — the 2-pound cut is more concentrated and produces a thicker film.