Why and how are surfaces treated and finished after a product is made?
The reasons for applying surface treatments and finishes (protection, durability, aesthetics, hygiene) and named finishes for timber, metal and polymer such as varnish, paint, galvanising, anodising, powder coating and self-finishing.
A focused answer to WJEC A-Level Design and Technology Unit 1 surface treatments and finishes, covering why finishes are applied (protection, durability, aesthetics, hygiene) and named finishes for timber (varnish, stain, oil), metal (galvanising, anodising, powder coating, electroplating) and polymers (self-finishing).
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What this dot point is asking
WJEC wants you to explain why a finish is applied and to name and describe appropriate finishes for timber, metal and polymers. The exam reliably asks you to justify a finish for a specific product (a garden gate, a kitchen worktop, a child's toy) and to describe how a named finish such as galvanising or anodising actually works. You need the reasons and the named processes, with the right finish matched to the right material.
The answer
Why finishes are applied
A finish is rarely applied for one reason alone; a painted gate is both protected and coloured. Surfaces are usually prepared first (cleaned, degreased, sanded, primed) because a finish only performs if it adheres to a sound surface.
Timber finishes
- Varnish - a clear protective film that resists moisture and wear while showing the grain.
- Paint - an opaque coloured film, applied over a primer and undercoat, that protects and colours.
- Stain - colours the timber without hiding the grain; usually sealed with a topcoat.
- Oil (such as teak or Danish oil) - soaks in to nourish and water-repel, used on worktops and outdoor furniture.
- Wax - a soft sheen and light protection, easily renewed.
- Preservative - protects exterior timber against rot, insects and weather.
Metal finishes
- Painting - over a primer; cheap colour and corrosion protection.
- Galvanising - a zinc coating (hot-dip or electro), used on ferrous metal. Zinc is more reactive than iron, so it protects sacrificially, corroding in preference to the steel even where scratched.
- Anodising - electrolysis thickens the natural oxide layer on aluminium into a hard, integral, corrosion-resistant surface that can be dyed.
- Powder coating - a dry polymer powder sprayed onto an earthed part and oven-cured into a tough coloured film; weatherproof and solvent-free.
- Electroplating - a thin metal layer (chrome, nickel) deposited by electrolysis for protection and appearance.
- Lacquering - a clear coat sealing polished metal (such as brass) against tarnish.
Polymers: self-finishing
This is a real advantage of polymers, removing a whole production step. Polymers may still be printed or textured for branding.
Examples in context
Example 1. A chrome-plated tap. The brass body is electroplated with nickel then chrome for a hard, bright, corrosion-resistant and hygienic surface that brass alone would not keep, showing protection, durability and aesthetics combined in one finish.
Example 2. An anodised aluminium drinks bottle. Anodising thickens the oxide layer into a tough, integral, dyeable surface that will not chip or peel like paint, so the bright colour survives daily knocks, which is why premium bottles are anodised rather than painted.
Try this
Q1. State two reasons, other than appearance, for applying a finish to a product. [2 marks]
- Cue. Protection against corrosion or moisture; durability and wear resistance; hygiene and easy cleaning.
Q2. Explain why galvanising continues to protect steel even after the surface is scratched. [2 marks]
- Cue. Zinc is more reactive than iron, so it corrodes sacrificially in preference to the exposed steel, continuing to protect it.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20194 marksExplain why a mild steel garden gate is galvanised, and describe how galvanising protects the metal.Show worked answer →
A mild steel garden gate is exposed to rain and air, so it would rust (corrode) and weaken without protection. Galvanising coats the steel with a layer of zinc, usually by dipping the cleaned gate into molten zinc (hot-dip galvanising).
The zinc layer protects in two ways. First, it forms a physical barrier that keeps oxygen and water away from the steel. Second, and importantly, zinc is more reactive than iron, so even if the coating is scratched the zinc corrodes in preference to the steel, protecting it sacrificially. This is why galvanising keeps working even when the surface is damaged.
Markers reward the reason (prevent rusting of exposed ferrous metal), the method (zinc coating, hot dip), and at least the barrier protection, with extra credit for the sacrificial action of the more reactive zinc.
WJEC 20214 marksCompare anodising and powder coating as finishes for an aluminium product, giving one advantage of each.Show worked answer →
Anodising uses electrolysis to thicken aluminium's natural oxide layer into a hard, durable, corrosion-resistant surface that is part of the metal itself, so it does not chip or peel. It can be dyed in bright colours and gives a quality metallic finish, ideal for items such as phone bodies and bike parts.
Powder coating sprays a dry polymer powder onto the earthed product, which is then cured in an oven to flow into a tough, even, coloured plastic film. It gives a thick, hard-wearing, weatherproof coloured coating in a wide colour range, with no liquid solvents, and covers a range of metals.
A good answer gives one advantage of each: anodising is integral so cannot peel and resists wear; powder coating gives a thick weatherproof coloured layer with low solvent use. Markers reward a genuine point of comparison plus a valid advantage for each.
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Sources & how we know this
- WJEC AS/A Level Design and Technology specification — WJEC (2017)