Why are products finished, and which treatment suits each material?
Surface treatments and finishes applied for functional and aesthetic purposes, including finishes for timbers, metals and polymers, and how they protect and improve products.
A focused answer to AQA GCSE Design and Technology specialist principle on surface treatments and finishes, covering finishes for timbers, metals and polymers and their functional and aesthetic purposes.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
This is AQA section 3.2.7. AQA wants you to know why surfaces are treated and finished, for both function and appearance, and which finish suits timber, metal or polymer. You need to name appropriate finishes and explain what they protect against and how they improve a product. In Paper 1 this is examined by asking you to choose and justify a finish for a named material and product.
Why we finish products
Before any finish is applied the surface must be prepared: timber is sanded smooth, metal is filed and degreased, so the finish bonds well and looks even. A poorly prepared surface lets a finish flake or peel.
Finishes for timber
Timber is porous and rots if moisture gets in, so an outdoor wooden product (a bench, a gate) needs a tougher, weatherproof finish such as exterior varnish or paint, while an indoor table may use oil or wax for a natural look that is easy to top up.
Finishes for metal
- Painting and powder coating: add colour and a protective barrier; in powder coating a polymer powder is sprayed on and cured in an oven to a tough, even film, common on bike frames and appliances.
- Galvanising: dips steel in molten zinc; the zinc corrodes in preference to the steel (sacrificial protection), giving long-lasting outdoor rust protection.
- Anodising: thickens the natural protective oxide layer on aluminium by electrolysis and allows it to be dyed a colour, used on phones and cookware.
- Dip coating (plastic dip): heats the metal and dips it in fluidised polymer powder to coat it in a tough, grippy plastic layer, used on tool handles and dish racks.
Ferrous metals (which contain iron) rust readily, so they need the most protection; non-ferrous metals such as aluminium resist corrosion better but are still often finished for appearance.
Finishes for polymers
Polymers are usually self-coloured during moulding and naturally resist corrosion and moisture, so they often need no protective finish, which is a manufacturing advantage. Surfaces can still be polished, printed, textured in the mould, or have a vinyl decal added for appearance, branding or grip.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20204 marksA mild steel garden gate needs finishing. Explain why it must be finished and describe two suitable finishes.Show worked answer →
A 4-mark question gives marks for the reason and for two appropriate, explained finishes.
Reason: mild steel is a ferrous metal, so it rusts (corrodes) when exposed to moisture and air outdoors. A finish is needed to seal the surface and stop oxygen and water reaching the steel, and also to give colour and improve appearance.
Finish 1, galvanising: the steel is dipped in molten zinc, which forms a protective coating that corrodes in preference to the steel (sacrificial protection), giving long-lasting outdoor rust protection. Finish 2, powder coating or painting: a coloured polymer or paint layer is applied and cured to form a tough barrier against moisture and to provide an attractive colour.
Markers reward (1) the reason (ferrous steel rusts outdoors), (2) and (3) two suitable named finishes each with how it protects. A finish that does not suit steel (such as varnish for wood) earns nothing.
AQA 20183 marksExplain the difference between the functional and aesthetic purposes of a finish, using a timber product as an example.Show worked answer →
A 3-mark Explain wants both purposes illustrated.
A functional finish protects the material, for example varnish or oil on a wooden table seals the timber against moisture, stains and wear, so it lasts longer and does not absorb spills. An aesthetic finish improves appearance, for example a wood stain deepens the colour and enhances the grain so the table looks more attractive, while a high-gloss varnish adds sheen.
Many finishes do both at once: varnish on a table both seals (functional) and adds gloss (aesthetic). Markers reward (1) functional means protection (moisture, wear), (2) aesthetic means appearance (colour, gloss, grain), (3) a timber example showing each. Giving only one purpose caps the mark.
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Sources & how we know this
- AQA GCSE Design and Technology (8552) specification — AQA (2017)