DT18: Finishing Techniques
Surface finishes for metals, polymers, timber and textiles; painting, powder coating, anodising, plating, varnishing, laminating; functional and decorative purposes.
Finishing Techniques
Surface finishes for metals, polymers, timber and textiles; painting, powder coating, anodising, plating, varnishing, laminating; functional and decorative purposes.
Key Fact: Metal finishes - functional: galvanising (zinc coating prevents rust), anodising (thickened aluminium oxide layer - harder, can be dyed), plating (chromium/nickel for hardness). Decorative: powder coating (baked polymer - durable, many colours), painting (versatile but less durable).
Key Fact: Timber finishes - functional: preservative (prevents rot), fire retardant. Decorative: varnish (clear, highlights grain, waterproof), oil (penetrates wood, natural), wax (soft sheen, less durable), paint (opaque, hides grain), stain (changes colour, shows grain).
Key Fact: Polymer finishes - self-finishing (coloured in the mould). Surface: printing (pad, screen, hot-foil), texturing (etched mould surfaces), vacuum metallising.
Key Fact: Textile finishes - functional: waterproofing (DWR coating), flame retardancy, stain resistance. Decorative: dyeing, printing, embroidery, applique.
Key Fact: Finishing purpose: protection (corrosion, UV, abrasion, moisture), identification (branding, safety warnings), aesthetics (colour, texture, sheen).
📋 Key Vocabulary and Concepts
For Finishing Techniques, you must know:
- Powder coating
- Galvanising
- Anodising
- Electroplating
- Varnish
- Oil finish
- Wax finish
- Stain
- Laminate
- Vacuum metallising
- Pad printing
- Screen printing
- Hot-foil stamping
- DWR coating
- Flame retardant
- Self-finishing
❓ Practice Questions
Q1: Explain why galvanising is used on steel gate hinges but not on stainless steel hinges.
Q2: Compare varnish and oil as finishes for an oak dining table, justifying your recommendation for a family home.
Q3: A polymer product needs the company logo applied. Compare pad printing and screen printing.
✅ Answers
- Steel contains iron which rusts. Galvanising coats with zinc - zinc corrodes sacrificially, protecting exposed iron even if scratched. Stainless steel has 10.5%+ chromium forming a self-healing oxide layer - does not rust, so galvanising is unnecessary.
- Varnish: hard clear film - excellent waterproof protection, resists scratches and heat, easy to wipe clean. Oil: penetrates wood, warm natural look, easy to repair, water-resistant but not waterproof, needs regular reapplication. For a busy family home: varnish for superior protection against spills and scratches.
- Pad printing: transfers ink via silicone pad - excellent for curved/irregular surfaces, small to medium logos, fine detail. Screen printing: ink through mesh stencil - best for flat surfaces, large areas, bold colours. For curved polymer casing: pad printing. For flat panel: screen printing.
🎯 Exam Tips
- State WHY the finish is needed (protection/aesthetics/identification) before choosing
- Link the finish to the material AND the use environment
- Know which finishes are decorative vs functional vs both
📝 Exam Technique
D&T Exam Tips:
For finishing questions: 1) Identify the material, 2) State requirements, 3) Propose an appropriate finish, 4) Explain how it works, 5) Justify the choice.
For finishing questions: 1) Identify the material, 2) State requirements, 3) Propose an appropriate finish, 4) Explain how it works, 5) Justify the choice.
⚠️ Common Errors
Watch Out!
Students often make mistakes here. Wrong: Painting is always the best finish because it gives the most colour options and covers any surface. Correct: Paint is versatile but has limitations: it chips and scratches easily, hides the material beneath, requires surface preparation and regular maintenance. For a garden gate, galvanising + powder coating is far more durable. For an oak table, varnish protects while showing the grain.
✍️ Model Answer
Full-Mark Response
A manufacturer produces aluminium outdoor furniture. Evaluate THREE appropriate finishing techniques and recommend the most suitable for a coastal environment.
A grade 9 response will: identify coastal challenges (salt spray, UV, moisture); evaluate anodising (excellent corrosion resistance, UV stable, oxide layer is integral to aluminium - cannot chip, self-healing); powder coating (good but can chip on edges); painting (least durable - salt penetrates cracks, paint peels). Recommendation: anodising is most suitable for coastal environments.
📊 AO Deep Dive
Assessment Objective Analysis
AQA D&T 8552: Written exam 50% + NEA 50%. AOs: AO1 Recall (20%), AO2 Apply (30%), AO3 Analyse & evaluate (50%). For grade 9, demonstrate detailed knowledge of finishing processes and perceptive evaluation of finish suitability for specific environments.
📝 Exam Questions by Topic
🎬 Video Resources
Detailed Notes
Surface Preparation and Coating Types
Surface preparation is critical for any finishing process because coatings adhere poorly to contaminated, oxidised or rough surfaces. For metals, preparation includes degreasing (solvent or alkaline cleaning), abrasive removal of oxide layers, and etching for improved adhesion. For timber, preparation involves sanding through progressive grits (120, 180, 240) to create a smooth surface, filling any grain or defects, and ensuring moisture content is below 12% to prevent subsequent shrinkage. For polymers, surface treatment such as flame treatment or plasma etching may be required to improve paint adhesion, particularly on polyethylene and polypropylene surfaces which are chemically inert.
Finishes serve functional and aesthetic purposes. Paint provides colour and protection, available as water-based acrylics (low VOC, quick-drying, UK school-safe) and solvent-based enamels (harder finish, higher VOC, requiring ventilation). Powder coating applies dry polymer powder electrostatically, then cures in an oven to create a tough, uniform finish used extensively in UK metal furniture and appliance manufacturing. Varnish and lacquer protect timber with clear coatings, with polyurethane varnish providing the hardest wearing surface for UK domestic woodwork. Oil finishes penetrate the wood, enhancing the natural grain whilst providing water resistance, with Danish oil and linseed oil being popular choices in British woodwork.
Example
A student finishing a mild steel outdoor bench specifies hot-dip galvanising as the primary protection because the zinc coating (applied by dipping the steel in molten zinc at 450 degrees Celsius) provides both barrier protection and sacrificial cathodic protection. Even if the coating is scratched, the zinc corrodes preferentially, protecting the underlying steel. They then apply a powder-coated top layer for colour and additional barrier protection, creating a dual-layer defence system suitable for the UK's wet climate.
Metal Finishing Processes
Metal finishing encompasses processes that alter the surface to improve appearance, corrosion resistance or wear properties. Electroplating deposits a thin layer of metal (chromium, nickel, zinc, gold) onto a conductive substrate using an electrolytic cell. Chrome plating provides a hard, reflective surface used on UK bathroom fittings and car bumpers, though environmental concerns about hexavalent chromium are driving adoption of trivalent chromium processes. Anodising thickens the natural oxide layer on aluminium, providing a hard, corrosion-resistant surface that can be dyed in a range of colours. British company Luxfer uses anodised aluminium in its high-pressure gas cylinders.
Other metal finishing techniques include polishing (mechanical abrasion to create a smooth, reflective surface), shot peening (blasting with small metal balls to induce compressive surface stress, improving fatigue resistance in UK aerospace components), and plating for electronic contacts. Gold plating on PCB edge connectors provides corrosion-free electrical contact. Tin plating on food cans prevents steel from contaminating contents. Hot-dip galvanising provides thick zinc coatings for structural steelwork, widely used in UK construction for beam connections and outdoor structures. GCSE students must select finishes appropriate to the operating environment, material and aesthetic requirements of their product.
Example
A student designing aluminium bicycle handlebars specifies hard anodising (Type III, 25-75 micron oxide layer) rather than standard anodising because the thicker coating provides significantly greater wear resistance at the grip contact area, where friction from the rider's hands would quickly wear through a thinner coating. The anodised surface is then dyed black to meet the aesthetic specification.
Timber and Polymer Finishing
Timber finishing enhances and protects the natural surface. Stains alter the colour whilst allowing grain to show through, useful for matching less expensive timbers to premium species. Wood dye penetrates deeper than stain for more uniform colour. Wax provides a natural, low-sheen finish suitable for UK antique furniture restoration and traditional woodwork. French polishing (multiple thin coats of shellac applied with a rubber pad) produces the highest-quality mirror finish on British fine furniture, though it is labour-intensive and easily damaged by water or alcohol. Modern alternatives include cellulose lacquers (spray-applied, quick-drying) and conversion varnishes (catalysed for extreme durability in commercial joinery).
Polymer finishing includes printing (pad printing for logos on moulded products, screen printing for flat surfaces), painting (using adhesion promoters and flexible paints that accommodate the polymer's thermal expansion), and surface texturing achieved through the mould tool finish. Polishing is used on acrylic edges to restore optical clarity after machining, using a flame polish technique or progressive abrasive polishing. Vacuum metalising deposits a thin aluminium layer on polymer surfaces to create chrome-like finishes on UK consumer products, including cosmetic packaging and automotive interior trim. GCSE students should specify the finishing process alongside the material choice, as the finish is integral to the product's overall performance and appearance.
Example
A student finishing an oak jewellery box applies a sanding sealer to fill the open grain, sands to 400 grit between coats, then applies three thin coats of polyurethane varnish with a natural bristle brush, sanding lightly between each coat. This produces a smooth, water-resistant finish that protects the oak from cosmetic oils and daily handling whilst showcasing the wood's distinctive medullary ray figure.
Comparison
Finishing Process Comparison
| Process | Material Suitability | Key Property | Durability | UK Use |
|---|---|---|---|---|
| Powder coating | Metals | Tough, uniform, colour range | Very high (10+ years) | Metal furniture, appliances |
| Anodising | Aluminium | Hard, can be dyed | High (5-10 years) | Window frames, consumer goods |
| Galvanising | Steel | Sacrificial corrosion protection | Very high (25+ years) | Outdoor structures, fencing |
| Polyurethane varnish | Timber | Hard wearing, water resistant | High (5-10 years) | Domestic woodwork, flooring |
| Danish oil | Timber | Enhances grain, water resistant | Medium (reapply yearly) | Furniture, worktops |
| Pad printing | Polymers | Logo/text on curved surface | Medium (3-5 years) | Consumer product branding |
Extended Practice
Q1: A student designing an outdoor steel bench for a UK park must specify a finishing system that will withstand British weather for at least 10 years. Compare hot-dip galvanising, powder coating and a combined system, justifying your recommendation with reference to barrier protection, sacrificial protection and cost.
Q2: Compare the suitability of polyurethane varnish, Danish oil and French polish for finishing an oak dining table, evaluating each with reference to durability, water resistance, ease of application and the visual quality of the finished surface.
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