Why are finishes applied to products, and how is a finish matched to the material and its use?
Select and apply appropriate surface finishes to woods, metals and plastics, explaining how finishes protect, improve appearance and suit the material
A focused answer to the O-Level Design and Technology outcome on finishing. Why finishes are applied (protection, appearance), finishes for wood, metal and plastic, and preparation before finishing.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
SEAB wants you to select and apply suitable surface finishes to woods, metals and plastics, and to explain how a finish protects the material, improves its appearance, and suits the material and its use. You should also understand surface preparation, because a finish only works on a properly prepared surface. The marks reward a finish correctly matched to the material and use, with reasoning.
The answer
Why finishes are applied
A finish is a coating or treatment applied to a product's surface. Finishes are applied for two main reasons:
- Protection. To guard the material against moisture, weather, corrosion, wear, dirt and decay, so the product lasts longer. Bare softwood rots outdoors; bare steel rusts.
- Appearance. To improve colour, sheen and texture, making the product more attractive and often easier to clean.
Many finishes do both at once: exterior varnish protects wood and enhances its grain. Choosing and applying a finish is a real part of making a quality product.
Finishes for wood
- Varnish. A clear or tinted coat that protects against moisture and wear while showing the grain; exterior grades suit outdoor use.
- Paint. An opaque coloured coat that protects and hides the grain; good on manufactured boards like MDF.
- Wood stain. Colours the wood while leaving the grain visible, usually sealed with varnish or oil.
- Oil and wax. Soak in to nourish and protect wood with a natural look, suiting furniture and tool handles.
- Preservative. Treats outdoor timber against rot and insects.
Finishes for metal
- Paint. Protects ferrous metals from rust and adds colour; often over a primer.
- Powder coating. A tough, durable coloured coating baked onto metal, common on outdoor furniture and appliances.
- Plating (e.g. chrome, zinc/galvanising). A metal coating that resists corrosion and can look bright; galvanising protects steel from rust.
- Lacquer. A clear coat protecting bright metals such as brass from tarnishing.
- Oil. Protects bare steel tools from rust in storage.
Finishes for plastics
Plastics are often self-finishing, because the colour and surface come from the moulding and the material does not corrode. Acrylic edges can be smoothed and polished to a clear shine. Finishing plastics is usually about smoothing and polishing rather than coating, though some plastics can be painted with suitable primers.
Surface preparation
A finish only works on a properly prepared surface. Preparation makes the finish adhere well and look smooth, even and durable; a rough, dirty, greasy or damp surface causes patchy, peeling finishes. For wood, sand smooth with progressively finer abrasive (with the grain), remove dust, and ensure it is clean and dry. For metal, remove rust and grease and prime if needed. Good preparation is most of a good finish.
Examples in context
Example 1. A galvanised steel roofing sheet. Steel rusts quickly in the weather, so roofing sheets are galvanised: coated with zinc that protects the steel from corrosion even if scratched. The finish is chosen purely for protection against rust in a harsh outdoor setting, dramatically extending the steel's life compared with leaving it bare or merely painted.
Example 2. An oiled wooden chopping board. A chopping board contacts food and water daily, so it needs a food-safe finish that resists moisture without flaking into food. Food-safe oil is rubbed in to nourish and water-resist the wood while keeping a natural surface, whereas varnish or paint could chip into food. The finish suits both the material (wood) and the hygienic, wet use.
Try this
Cue. State two reasons a finish is applied to a product. Answer: to protect the material (from moisture, corrosion, wear or decay) and to improve its appearance.
Cue. Recommend a finish for an outdoor steel railing and say why. Answer: powder coating or paint over a primer (or galvanising), because it seals the steel against moisture and prevents rust while adding colour.
Cue. Explain why acrylic usually needs no protective finish. Answer: it does not corrode and its colour is in the material, so finishing is limited to smoothing and polishing cut edges rather than applying a protective coating.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksA designer has made a pine garden planter. (a) Give two reasons why a finish should be applied. (b) Recommend a suitable finish for the planter and explain how it suits the material and its use.Show worked answer →
(a) Two reasons: to protect the wood from moisture, weather and decay so it lasts longer outdoors; and to improve its appearance (colour, sheen) and make it more attractive and easier to keep clean.
(b) Suitable finish: an exterior wood preservative or exterior varnish/paint. It suits the material and use because pine is a softwood that absorbs water and rots outdoors if left bare, so a weatherproof preservative seals it against moisture and fungal decay, greatly extending its life in a wet, outdoor planter, while also improving its look. Preparation (sanding smooth, clean and dry) is needed first for the finish to adhere.
What markers reward: two genuine reasons (protection and appearance), and a finish correctly matched to softwood used outdoors (a weatherproof preservative/varnish or paint), with reasoning about moisture and decay and a note on surface preparation.
Original4 marksExplain why a surface must be properly prepared before a finish is applied, and describe one way to prepare a wooden surface.Show worked answer →
A surface must be properly prepared so the finish adheres well and gives a smooth, even, long-lasting result. If the surface is rough, dirty, greasy or damp, the finish will not stick properly, may look uneven or patchy, and can peel or flake off, wasting the finish and leaving the material unprotected.
To prepare a wooden surface, sand it smooth with progressively finer abrasive paper (working with the grain), then remove all dust and ensure it is clean and dry before applying the finish. This gives a smooth, clean, dry surface for the finish to bond to.
What markers reward: the point that preparation lets the finish adhere and gives a smooth, durable, even result and stops it peeling, plus a correct preparation method for wood (sand smooth with the grain, remove dust, ensure clean and dry).
Related dot points
- Mark out and measure work accurately using rules, squares, gauges, dividers and templates, and explain why accuracy and datum surfaces matter
A focused answer to the O-Level Design and Technology outcome on marking out. Rules, try squares, marking gauges, dividers and templates, the use of a datum, and why accuracy matters.
- Select and use appropriate cutting, shaping and forming processes for woods, metals and plastics, including sawing, drilling, filing and line bending, and work safely
A focused answer to the O-Level Design and Technology outcome on processes. Sawing, drilling, filing, abrading and forming such as vacuum forming and line bending, matched to the material, with safety.
- Select and use appropriate joining methods, including adhesives, mechanical fixings and wood joints, and distinguish permanent from temporary (knock-down) joints
A focused answer to the O-Level Design and Technology outcome on joining. Adhesives, mechanical fixings, wood joints and knock-down fittings, and choosing between permanent and temporary joints.
- Classify woods as hardwoods, softwoods and manufactured boards, describe their properties and uses, and select a wood for a given application
A focused answer to the O-Level Design and Technology outcome on woods. Hardwoods, softwoods and manufactured boards (plywood, MDF, chipboard), their properties, and choosing the right wood.
- Classify metals as ferrous, non-ferrous and alloys, describe their properties and uses, and explain why alloys are made
A focused answer to the O-Level Design and Technology outcome on metals. Ferrous and non-ferrous metals, alloys such as steel and brass, their properties and uses, and why alloys are made.