Galvanising vs Powder Coating: Choosing a Finish for Steel

The finish on fabricated steel decides how long it lasts and what it looks like. The main choices are hot-dip galvanising, powder coating, the two together (duplex), a paint system, or stainless steel with no coating at all. Here's how each one works and where it makes sense.

The short answer

• Outside, and not on show: hot-dip galvanised.
• Outside, and on show or in a colour: galvanised, then powder coated (duplex).
• Inside, on show: powder coated in your colour, or stainless.
• Inside, hidden in the structure: often just a primer, or a paint system the engineer specifies.

Hot-dip galvanising

The steel is cleaned in a series of chemical baths, then dipped into molten zinc at around 450°C. The zinc bonds to the steel and forms a tough coating over every surface, inside and out, including edges and corners that are hard to reach with paint. The standard is BS EN ISO 1461.

Galvanising protects in two ways. The zinc is a barrier, and it's also sacrificial: where the coating is scratched, the zinc around the scratch corrodes instead of the steel. That's why galvanised steel copes with knocks and scrapes far better than paint.

• Looks: a metallic grey that dulls with age. Practical rather than decorative.
• Lifespan: commonly several decades in typical UK conditions, less on the coast or in aggressive industrial atmospheres.
• Maintenance: very little.

Designing for galvanising

Galvanising has to be planned at the fabrication stage, not added as an afterthought:

• Vent and drain holes. Hollow sections need holes so zinc can flow in and out and trapped air can escape safely in the bath.
• Size. The piece has to fit the galvaniser's bath, which can decide where a long assembly is split.
• Distortion. Thin plate and large flat panels can warp in the heat.
• Welding afterwards. Welding on site burns off the zinc, so the area needs repairing with a suitable zinc-rich coating. It's better to bolt galvanised parts together on site.

Powder coating

A dry powder is sprayed onto the steel with an electrostatic charge so it clings evenly, then baked in an oven, where it melts and cures into a hard, even film.

• Looks: any RAL colour, in gloss, satin, matt or textured finishes. Much harder-wearing than wet paint.
• Best for: internal staircases, balustrades, handrails, screens, furniture and fittings.
• The catch: over bare steel outdoors, once the coating is chipped, rust can creep in underneath it. For external work it's best on top of galvanising.

Good preparation is what makes powder coating last: the steel is blast cleaned and pre-treated before the powder goes on.

Duplex: galvanised and powder coated

Galvanise first, then powder coat. You get the colour and finish of powder coating, with galvanising underneath to protect the steel if the top coat is ever damaged. A duplex system lasts longer than either coating would on its own.

It's the usual choice for external steel that people see and touch: balustrades and railings, external fire escapes in a building's colours, canopies, gates and shopfronts.

The galvanised surface needs specific preparation before it's powder coated so the coating bonds properly, which is something the coater handles.

Paint systems

Structural steel is often painted rather than galvanised or powder coated, using a primer and, where needed, further coats. Paint systems are specified to ISO 12944, which grades environments from C1 (heated, dry interiors) up to C5 and CX (very aggressive, such as marine and heavy industrial).

Steel hidden inside a warm, dry building is in the mildest category and often needs little more than a primer. Where a beam needs fire protection with intumescent paint, the primer has to be compatible with it. See steel beam fire protection.

Stainless steel

Stainless doesn't need a coating at all. It's finished brushed or polished and is common in hospitality for bar fittings, glass racks and handrails, and wherever hygiene matters. Grade 304 suits most interiors; grade 316 is the usual choice outdoors and near the coast. It costs more than coated mild steel, so it's normally used where it's on show or where cleaning matters.

Side by side

Finish	Colour	Outdoors	If it's scratched	Typical use
Hot-dip galvanised	Metallic grey	Excellent	Zinc protects the scratch	Fire escapes, walkways, structural steel outside
Powder coated	Any RAL	Fair on bare steel	Rust can creep under the coat	Internal stairs, balustrades, screens
Duplex	Any RAL	Excellent	Galvanising protects the steel	Visible external steelwork
Paint system	Usually limited	Depends on the system	Needs touching up	Structural steel, beams
Stainless steel	Brushed or polished	Excellent (316)	Nothing to repair	Bar fittings, handrails, food areas

What to tell your fabricator

When you ask for a quote, it helps to say:

• Where it lives: inside or outside, and how close to the coast or anything corrosive
• Whether it's on show, and the colour or finish you want
• Whether it needs fire protection
• How it will be fixed on site, so galvanised parts can be bolted rather than welded

We specify finishes as part of every job. External steelwork is hot-dip galvanised, and where it's on show we galvanise and then powder coat to your RAL colour. See our bespoke metalwork and staircases and fire escapes services, or get in touch for a quote.

Frequently asked questions

Is galvanised or powder coated better? For outdoor protection, galvanised. For colour and appearance, powder coated. For outdoor steel that has to look good, both together.

Can you powder coat galvanised steel? Yes. That's a duplex finish. The galvanised surface needs preparing first so the powder coat bonds properly.

Can you paint galvanised steel? Yes, with the right preparation and a primer suited to zinc. Ordinary paint applied straight onto new galvanising tends to peel.

Does galvanised steel rust? Eventually, but slowly. The zinc wears away over many years before the steel underneath starts to corrode. New galvanising stored wet can show a white powdery bloom, which is a surface effect, not rust.

How long does powder coating last? Indoors, many years with little more than cleaning. Outdoors, it lasts far longer over galvanising than over bare steel.

Can galvanised steel be welded? It can, but welding burns off the zinc and gives off fumes, so the weld area has to be repaired afterwards. Where possible, galvanised parts are designed to be bolted together on site.