1.1.27 Surface Finishes & Treatments
D&T 2026 — 1.1(c) Product Design Pathway
Why surface treatments and finishes are applied
Surface treatment — anything applied to, or done to, the surface of a material to protect it, improve its appearance, or both.
Finishes have two jobs, and a good choice usually does both.
Prolonging material life (functional):
- Metals — ferrous metals rust when exposed to air and moisture; a finish seals the surface so oxygen and water cannot reach it.
- Timbers — wood absorbs moisture, so it swells, warps, rots and stains. A finish seals it and can protect against insects, fungi and UV light.
- Polymers — most do not corrode, but UV light can make them fade and go brittle, and scratches spoil clear surfaces.
- Finishes also make products hygienic and easy to clean, more hard-wearing, and can add grip or insulation.
Improving aesthetic appeal:
- Adds colour, which can match a brand, a colour scheme or a target market.
- Changes texture and sheen — high gloss, satin or matt.
- Enhances or hides the natural features of a material, such as wood grain.
Preparation comes first. A finish is only as good as the surface beneath it. Metals must be cleaned of rust, grease and scale (with emery cloth, wire wool or degreaser); timber must be sanded smooth with progressively finer abrasive paper and the dust removed. Some finishes need the surface keyed (lightly roughened) so they grip.
Metal surface treatments: powder and plastic coating, paint and primer
| Finish | How it is applied | Benefits | Typical uses |
|---|---|---|---|
| Powder coating | Dry polymer powder is sprayed on with an electrostatic charge so it clings to the metal, then baked in an oven so it melts into a hard, even coat | Tough, chip-resistant and durable; wide range of colours and textures; no solvents, and overspray can be reused | Bicycle frames, garden furniture, car wheels, radiators, appliance casings |
| Plastic (dip) coating | The metal is heated (about 200°C) and dipped into a fluidised bed of polythene powder, which melts onto it; it is then returned to the oven to cure | Thick, soft, warm and grippy; good electrical and thermal insulation; protects against corrosion | Tool handles, dish racks, wire baskets, garden tools, playground equipment |
| Paint and primer | A primer is applied first, then one or more top coats by brush, roller or spray | Cheap and simple; any colour; easy to touch up | Gates, railings, machinery, vehicles, school metalwork |
Primer — the first coat applied to bare material. It seals the surface, prevents corrosion (on metal) and helps the top coat stick. Ferrous metal needs a metal primer; paint applied straight onto bare steel will soon flake and rust underneath.
Protecting natural and manufactured timbers
| Finish | How it works | Benefits and drawbacks | Typical uses |
|---|---|---|---|
| Varnish (e.g. polyurethane) | A clear or tinted coating that dries into a hard protective layer; applied in thin coats, lightly sanded between coats | Hard-wearing and waterproof; gloss, satin or matt; shows the grain. Can chip, and is hard to repair invisibly | Floors, tables, chairs, doors, children’s toys |
| Wax | Rubbed into the surface and buffed to a soft sheen | Enhances the grain and feels natural; easy to apply and renew. Offers little protection against water or heat and needs reapplying | Fine furniture, turned bowls, wooden handles |
| Paint | Applied over a primer and undercoat, in one or more top coats | Any colour; hides the grain and defects; protects well. Hides the natural beauty of the timber | Children’s furniture, MDF products, skirting boards, outdoor joinery |
| Sealants | A sanding sealer soaks into and seals the pores of the wood before varnish, wax or paint | Fewer top coats are needed and the finish goes on evenly | Bare timber and MDF before finishing |
| Primers | Seal bare wood so paint does not soak in, and help the top coat stick | Essential on MDF, whose cut edges are very absorbent | All painted timber and manufactured boards |
Natural timber is often given a clear finish (varnish or wax) to show off its grain. Manufactured boards such as MDF and chipboard have no attractive grain, so they are usually painted, veneered or covered with a laminate (e.g. melamine on kitchen worktops). Outdoor timber needs a preservative or exterior-grade finish to resist rot and insects.
For a “describe the stages” question, give them in order: sand smooth with progressively finer grades, remove dust, apply sealer or primer, lightly sand, then apply thin top coats, sanding lightly between each.
The self-finishing nature of plastics
Self-finishing — a material that needs no applied finish, because it comes out of the manufacturing process with its final colour, texture and surface.
Many thermoforming and thermosetting plastics are self-finishing:
- Colour runs all the way through — pigment is added to the polymer before it is moulded, so scratches do not reveal a different colour underneath and there is no paint to chip off.
- The mould gives the surface — a polished mould produces a high-gloss product; a textured mould produces a matt, leather-effect or grip pattern. Every product made in that mould has the same finish.
- They do not corrode — so they need no protective coating against water.
This saves time, money and materials in manufacture, and is one reason injection-moulded products are so cheap in high volumes.
When plastics do need finishing: parts cut from stock sheet have rough edges, which are finished by draw filing, wet-and-dry paper (used wet), polishing compound or flame polishing (acrylic). Plastics can also be painted (with a suitable primer), printed, vinyl-decorated or laser-engraved for decoration or branding.