Surface Finishing

Why Surface Finish Matters

A surface finish does two things simultaneously: it protects the base material from corrosion and it gives the product its appearance. Both matter commercially: a rusted chassis is structurally weakened, and an ugly surface finish damages a brand. Surface finishing is the last manufacturing stage before a product leaves the factory, so faults here affect every unit that ships. Understanding which finish suits which material, environment and cost envelope is a key engineering decision.

Surface Preparation

Every surface finishing process begins with preparation. Paint, dip coatings and electroplating all bond poorly to a surface contaminated with grease, rust, mill scale or dust. Standard preparation steps are: degrease (solvent or alkali wash), abrade (grit-blast or sandpaper to provide a mechanical key), rinse and dry. Skipping preparation is the most common cause of coating failure in industry. A well-prepared surface can make a cheap paint finish outperform an expensive electroplated coating applied to a dirty surface.

Painting and Spray Painting

Painting applies a liquid coating to the surface that dries or cures to form a continuous protective film. Spray painting uses compressed air to atomise paint into a fine mist that settles evenly across the surface. It is used on vehicle bodywork, metal furniture, machinery guarding and architectural steelwork. Multiple thin coats are better than one thick coat, which sags and cures unevenly. High-volume industrial lines use electrostatic spray guns that charge the paint droplets so they are attracted to the earthed metal workpiece, reducing overspray waste.

Advantages of painting: wide colour choice; low cost for low-volume work; can be applied on-site to large structures (e.g. the Forth Road Bridge). Limitations: can chip or scratch to expose bare metal, allowing corrosion to undercut the surrounding paint (the most common failure mode); requires ventilation and personal protective equipment (PPE) due to solvent fumes.

Dip Coating

Dip coating (sometimes called plastisol dipping or powder dip coating) involves heating the cleaned metal component to around 200 °C and then dipping it into a fluidised bed of thermoplastic powder (typically PVC or polyethylene). The hot metal melts the powder particles that contact it, building up a thick, rubbery coating. The component is then cured in an oven. Dishwasher racks, tool handles, garden-chair frames and cycle-basket wire forms are typical applications: the thick thermoplastic coating gives grip, insulates electrically, resists knocks and provides complete waterproofing.

Advantages over spray paint: the coating is much thicker (typically 1–3 mm vs 50–150 μm for paint), more chemically resistant, and resistant to impact without chipping. Limitations: colour range is narrower than paint; the process requires heating the component, so large structures cannot be dip-coated easily; it is more expensive than painting per unit for large flat surfaces where thin coverage is acceptable.

Electroplating

Electroplating uses electricity to deposit a thin, uniform layer of a metal onto the workpiece surface. The component to be plated is made the cathode (negative electrode) in an electrolytic cell; the plating metal (e.g. chromium, nickel, gold, copper, zinc) forms the anode (positive electrode). Current flows through the electrolyte solution, dissolving metal from the anode and depositing it on the cathode surface. Chromium plating on car bumpers and taps gives a mirror finish and excellent corrosion resistance. Gold plating on electrical contacts reduces contact resistance and prevents oxidation. Zinc electroplating (electrogalvanising) protects steel fasteners.

Advantages: very thin (1–50 μm), dimensionally precise coating that adds almost no thickness to machined surfaces; excellent surface finish and appearance; can deposit expensive metals (gold, platinum) very economically because the layer is so thin. Limitations: the plating bath and anodes must be managed carefully; wastewater from plating is chemically hazardous and requires specialist disposal; not suitable for complex internal cavities where current density is uneven.

Galvanising

Galvanising coats steel with a layer of zinc. The dominant method is hot-dip galvanising: clean, fluxed steel is submerged in a bath of molten zinc at approximately 450 °C. The zinc reacts with the steel to form a series of intermetallic alloy layers, bonded much more strongly than a paint film. Street furniture, motorway crash barriers, agricultural machinery and structural steelwork are galvanised as standard.

Galvanising has one property that paint lacks: sacrificial (cathodic) protection. Zinc is more electrochemically active than iron; if the zinc coating is scratched through to bare steel, the zinc around the scratch corrodes preferentially, protecting the exposed iron. The zinc “sacrifices itself” to protect the steel. Paint has no such mechanism — a scratch allows moisture to reach the steel and corrosion spreads under the coating.

Polishing and Anodising

Polishing removes surface roughness mechanically — using abrasive compounds, buffing wheels or electropolishing — to produce a reflective, smooth surface. It is used on cutlery, surgical instruments, jewellery and any part where appearance or hygiene demands a smooth surface (rough surfaces trap bacteria). Polishing alone does not protect against corrosion but prepares the surface for electroplating or lacquering.

Anodising is most commonly used on aluminium. Aluminium naturally forms a thin oxide layer that protects it from further corrosion, but anodising deliberately thickens this oxide layer using electrolysis. The anodised surface is hard, scratch-resistant and porous: dye can be absorbed into the pores before sealing, giving a wide range of colours. Laptop casings, window frames, bicycle components and camera bodies are commonly anodised. Unlike paint or plating, the anodised layer is part of the aluminium itself and cannot peel.