Why are surface treatments and finishes applied, and which finish suits each material?
Surface treatments and finishes: why finishes are applied (protection, appearance, function), and suitable finishes for each material, including painting, varnishing and oiling timber, galvanising, anodising and powder coating metal, and self-finishing polymers.
A focused answer to Eduqas GCSE Design and Technology (C600) on surface treatments and finishes: why finishes are applied and suitable finishes for timber, metal and polymers, including galvanising, anodising and powder coating.
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What this dot point is asking
Eduqas C600 expects you to know why surface treatments and finishes are applied (protection, appearance, function) and which finish suits each material. You need finishes for timber (paint, varnish, oil, preservative), metal (galvanising, anodising, powder coating, painting) and polymers (usually self-finishing). In the written exam this is tested by explaining why a finish is needed and by comparing finishes such as galvanising and powder coating.
Why finishes are applied
The three reasons often overlap: an exterior varnish on a bench protects against rain while improving the look. Outdoor and high-wear products need protective finishes most; the surface must be properly prepared (cleaned, sanded, primed) for the finish to last.
Finishes for timber
Timber must be sealed against moisture, which causes rot, warping and greying.
- Paint: covers the grain with a coloured, protective coat (good for softwood and outdoor use).
- Varnish: a clear, hard-wearing coat that protects while showing the grain (interior or exterior grades).
- Oil (teak, Danish, linseed): soaks into the wood for a natural, low-sheen finish, easy to reapply.
- Preservative: protects exterior timber (fencing, decking) against rot and insects.
Finishes for metal
Metal is finished mainly to prevent corrosion and to add colour.
- Galvanising: coats steel with zinc (by dipping in molten zinc); the zinc forms a barrier and sacrificially corrodes instead of the steel, protecting it even if scratched (outdoor steel, fencing).
- Anodising: thickens the natural oxide layer on aluminium electrochemically, giving a tough, corrosion-resistant surface that can be coloured (phone bodies, trims).
- Powder coating: sprays a dry polymer powder that is baked into a tough, even, coloured coating (appliances, bike frames).
- Painting: a coloured protective coat, often over a primer to stop rust on steel.
Finishes for polymers
This is a major advantage of polymers: an injection-moulded part comes out coloured and smooth, saving the cost and time of a finishing stage. Polymers also do not rust or rot, so protection is rarely needed.
Try this
Q1. Name a finish suitable for protecting an aluminium product that can also add colour. [1 mark]
- Cue. Anodising (or powder coating).
Q2. State why most polymers do not need a separate finish. [1 mark]
- Cue. They are self-finishing: the colour and smooth surface are built in during moulding (and they do not rust or rot).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas C600 20183 marksExplain why a finish is applied to a softwood garden bench, and name a suitable finish.Show worked answer →
A 3-mark question: marks for the reasons (protection and appearance) and a suitable finish.
A finish is applied to a softwood garden bench mainly to protect it: outdoors it is exposed to rain and sun, and softwood would absorb water, rot and grey without protection. A finish seals the surface against moisture and UV, and it also improves appearance and makes the bench easier to clean.
A suitable finish is an exterior wood preservative, an exterior varnish or a wood oil (such as teak or Danish oil), which protect the timber while letting the grain show.
Markers reward the reasons (protect from moisture, rot and UV; improve appearance) and a suitable exterior timber finish. Naming an indoor-only or metal finish, or giving no reason, loses marks.
Eduqas C600 20214 marksExplain the difference between galvanising and powder coating as finishes for a steel product, including how each protects the metal.Show worked answer →
A 4-mark Explain wants both finishes described with how they protect.
Galvanising coats steel with a layer of zinc (usually by dipping in molten zinc). The zinc protects the steel by forming a barrier and by sacrificially corroding instead of the steel, so the steel does not rust even if the surface is scratched. It is hard-wearing and used for outdoor steel (gates, fencing).
Powder coating sprays a dry polymer powder onto the steel, which is then baked so it melts into a tough, even coloured coating. It protects by sealing the steel from moisture and air, and gives a durable, attractive coloured finish (appliances, bike frames).
Markers reward both finishes described with their protection mechanism (galvanising: a sacrificial zinc barrier; powder coating: a baked polymer seal) and a use. Confusing the two, or omitting how they protect, loses marks.
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Sources & how we know this
- WJEC Eduqas GCSE (9-1) Design and Technology (C600) specification — WJEC Eduqas (2017)