Surface finishes and coatings: what they do and when to use them

Why finish matters

A finish protects the part from rust and wear, makes it look right, and sometimes makes it work better, for example a smoother sliding surface. It is also a real cost, so say what you need in your requirement.

Common finishes

• Painting: gives colour and some protection. It is cheap and flexible, but can chip.
• Powder coating: a dry powder is sprayed and baked into a tough, even layer. It resists chips and scratches better than ordinary paint and suits steel and aluminium parts.
• Hot-dip galvanising: steel is dipped in molten zinc. It gives long protection against rust outdoors and suits structures. The surface is rougher and the part adds a little thickness.
• Electroplating (zinc, nickel, chrome): a thin metal layer is deposited on the part. It is used for small parts, fasteners and decorative or wear surfaces.
• Anodising: for aluminium, it makes the surface layer harder and more resistant to corrosion, and it can be coloured.
• Passivation and polishing: for stainless steel, to restore corrosion resistance and give a clean look.

Things to tell the workshop

Say which finish, what colour or standard, and where the part will be used. Mention which surfaces or holes must stay bare, such as threads and mating faces, because coatings add thickness. And ask who arranges the finish, because many workshops send parts to a specialist partner.

Kinds of work

• Die making
• Mold making
• CNC machining
• Casting
• Fabrication