Why do we apply finishes?
A finish (or surface treatment) is a layer or process applied to the outside of a material. It has two main purposes:
- Protection: stops corrosion (rust needs water + oxygen), rot and insects in wood, water absorption, fading from UV light, scratches and wear, stains and dirt, and in some cases fire.
- Aesthetics: adds colour, shine or a matt look, texture, a smooth or soft feel, or shows off natural grain.
Good surface preparation comes first: sanding wood, removing grease and rust from metal (degreasing, abrading). A finish on a dirty surface soon peels.
Finishes for woods and manufactured boards
- Paint: coloured, opaque and sealing; usually primer → undercoat → top coat. Hides the grain.
- Varnish / lacquer: clear, hard, waterproof coat (gloss, satin or matt); grain stays visible. Polyurethane varnish is very tough.
- Oil (e.g. teak or linseed oil) and wax: soak in or sit thinly, give a natural look and feel; need re-applying.
- Wood stain: changes colour, shows grain; gives little protection unless sealed with a clear coat.
- Preservatives: soak into outdoor timber (fences, sheds) to stop fungi and insects.
- Veneers and laminates on boards like MDF and chipboard: a thin layer of real wood or plastic for looks and wear resistance.
Finishes for metals
- Paint: a primer that sticks to metal, then top coats. Cheap, any colour, but chips can let rust start.
- Powder coating: dry plastic powder is sprayed onto charged metal, then baked so it melts into an even, tough layer. No solvents; used on bicycles, garden furniture, appliances.
- Galvanising: steel is dipped in molten zinc. Zinc reacts before iron does (sacrificial protection), so even a scratch does not rust quickly. Used for poles, barriers, roofing sheets, buckets.
- Electroplating: electricity deposits a thin layer of another metal (chrome, nickel, zinc, silver, gold) for shine and corrosion resistance; taps, cutlery, jewellery.
- Anodising (aluminium): an electrolytic process thickens the natural oxide layer, making it harder and corrosion resistant; it can be dyed in bright colours. Phones, torches, bottles.
- Dip coating (plastic dip on tool handles), polishing and oil/lacquer on bright steel are other options. Stainless steel and some alloys are self-finishing.
Finishes for polymers, papers and boards
Polymers are often self-finishing: pigment is mixed in and the mould gives the surface its gloss or texture, so no extra coat is needed. Extra finishes: polishing (cut edges of acrylic), printing (screen or pad printing of logos), vinyl decals and flame polishing.
Papers and boards: lamination (thin plastic film for strength and water resistance), UV varnish (glossy patches), embossing/debossing (raised or pressed shapes), foil blocking (metallic look). These add value to packaging and book covers but can make recycling harder.
Finishes for textiles
Fabric finishes change how a fabric looks, feels or performs.
- Mechanical: brushing/napping (soft, warm surface), calendering (pressed between hot rollers for shine), shrink-resist pre-shrinking.
- Chemical: water-repellent coatings, stain-resistant, flame-retardant (children's nightwear, curtains), anti-crease / easy-care, anti-bacterial, mothproofing.
- Laminated membrane fabrics: a very thin microporous film (often PTFE or polyurethane) is bonded between an outer fabric and a lining. Its pores are far smaller than a raindrop but larger than a water-vapour molecule, so the fabric is waterproof yet breathable. Used in rain jackets, walking boots and tents.
Some finishes wash out over time and some chemicals harm the environment, so newer finishes aim to be durable and safer.
Choosing a finish
Ask: (1) What is the material? (zinc and anodising suit metals; varnish suits wood). (2) Where is it used? (outdoors, wet, food contact, children). (3) What look is wanted? (4) Cost and scale (spray paint for one-offs, powder coating for batches). (5) Durability and maintenance. (6) Health, safety and the environment: low-VOC water-based paints, no toxic heavy metals, and finishes that do not stop recycling.
Try it
Walk round your home or school and find five different finishes: a painted wall, a varnished table, a galvanised pipe or pole (dull grey, sometimes with a crystal pattern), an anodised or powder-coated item, a laminated card. Put a drop of water on a varnished surface and on bare wood or cardboard (ask first). Which soaks in? Then try the last 3D step.
Key formulas and definitions
- Finish = Protection + Aesthetics
- Rust needs water + oxygen; a finish blocks them
- Galvanising = coating steel with zinc (sacrificial protection)
- Anodising = thickening aluminium's oxide layer by electrolysis
- Powder coating = sprayed powder + baking = tough even layer
- Laminated membrane = pores smaller than raindrops, bigger than vapour
Worked examples
1. Choose a finish for a steel garden gate and justify it.
Galvanising (or galvanising then powder coating). The zinc protects steel from rain and even protects scratched areas, so the gate lasts outdoors for many years with little care.
2. A designer wants an oak table to show its grain and resist spilled drinks. Which finish?
A clear polyurethane varnish or lacquer: it is transparent so the grain shows, and forms a hard waterproof layer.
3. Why is a phone's aluminium case often anodised rather than painted?
Anodising makes the surface oxide layer thicker and harder, so it resists scratches and corrosion, and it can be coloured. Paint would chip and wear off with handling.
4. Explain how a laminated membrane jacket can be waterproof but still let sweat out.
The membrane has tiny pores. Liquid raindrops are much bigger than the pores so they cannot pass. Water vapour from sweat is made of single molecules that are smaller than the pores, so it escapes.
5. Give one reason why many plastic products do not need a separate finish.
They are self-finishing: colour is added to the polymer and the mould gives a smooth, glossy or textured surface, saving time and cost.
Common mistakes
- Saying wood stain protects wood well. Stain mainly changes colour; it needs a varnish or oil on top for protection.
- Saying galvanising uses tin or chrome. Galvanising is a zinc coating.
- Thinking anodising works on steel. Anodising is mainly for aluminium (and some other non-ferrous metals).
- Forgetting surface preparation. Paint on greasy or rusty metal peels quickly.