Powder coating or hot dip galvanising: choosing a finish for outdoor steel
Powder coating and hot dip galvanising are not two grades of the same thing: one is a barrier film, the other is zinc that corrodes in place of the steel. This guide explains how each process works, when a duplex system is the right specification, what galvanising forces you to change in the design, and where stainless is the better answer. It ends with a comparison table and five questions that settle the choice before drawings are fixed.
Powder coating and hot dip galvanising get compared as if they were two grades of the same product. They are not: one is a film laid on the steel, the other is zinc that has reacted with the steel and become part of it. So the question is not which is better, but how corrosive the environment is where this piece will live, and how much colour matters to you.
What powder coating actually is
Powder coating is dry pigmented polymer powder, electrostatically charged, sprayed onto clean earthed steel and cured in an oven, where it melts into one continuous film. You get a closed skin in effectively any RAL Classic colour, with a choice of gloss and texture, from deep matt to a coarse structure. It is a barrier coating: it works by keeping water and oxygen off the steel. Degreasing, blasting and pretreatment matter more than the powder itself.
What hot dip galvanising actually is
Hot dip galvanising immerses the finished welded article in molten zinc at close to 450 degrees Celsius under EN ISO 1461. Zinc reacts with iron to create metallurgically bonded alloy layers and an outer zinc layer. That bond and sacrificial behaviour differ from a paint film, but they do not make every coating immune to flaking, local failure or damage. EN ISO 1461 acceptance and the actual condition remain decisive.
The mechanism difference that decides the answer
A barrier coating protects only where it is intact. A sacrificial coating protects the steel around the place where it is not.
Cut powder coating to bare steel and corrosion starts in the scratch, works under the film and lifts it, so a small mark spreads. Zinc is anodic to steel, so at a scratch, a hole or a cut edge the zinc corrodes first and keeps the exposed steel protected. An indoor table that nothing ever touches does not need that. A gate caught by a car door, shoved by a bin and clipped by a strimmer does, which is why heavy items in gates and fencing are usually galvanised.
Duplex: galvanise first, then powder coat
A duplex system adds a compatible colour coating over galvanising. It can protect the zinc from the environment and extend system durability, but the result depends on zinc preparation, coating compatibility, exposure, detailing and maintenance. Synergy is not unconditional.
The responsible coater must specify a compatible pretreatment and controls for contamination and outgassing. Cleaning, chemical conversion, sweep blasting or preheating may form part of that route, but no single recipe applies to every zinc surface and powder product.
What galvanising does to the design, before anything is welded
The finish decision changes the fabrication drawing, including hollow-section venting, drainage, weld layout, bath fit and any transport division.
Galvanising is not applied to a finished design. It constrains the design.
- Every closed section needs vent and drain holes so molten zinc can flood in and air can escape; moisture sealed into a hollow is dangerous in the bath. They sit diagonally opposite and stay visible unless the design hides them.
- The article has to fit the bath, whose size belongs to the galvanising plant. A piece slightly too long is dipped in two goes, leaving a visible line.
- Thin sheet and slender frames can distort, because the steel reaches bath temperature in minutes and rolling and welding stresses are released.
- Spangle, shade variation and local thickening can be normal visual features, but harmful roughness, uncoated areas and defects that affect use or conformity are not accepted merely as appearance. Inspect against EN ISO 1461 and the project requirements.
All of it is decided before the first weld, so the finish belongs in the order: a fabrication welded for paint cannot be galvanised later.
Where the piece will live
EN ISO 12944 grades atmospheres by corrosivity, from C1 for heated dry interiors to C5 for industry with near permanent condensation and for high salinity marine air. Used qualitatively it sorts most decisions.
- A garden well inland, on a piece that dries after rain: mild. Powder coating over well prepared steel suits garden furniture.
- A drive or street salted through the winter: aggressive low down, where the spray reaches. Galvanising or duplex.
- Within sight of the sea, or an indoor pool room with chlorine and warm condensation: severe, all year.
Chloride exposure does not create one universal instruction to use 316L stainless steel. Material grade, coating system, detailing, cleaning and the full chemical and temperature environment require project-specific selection by the responsible specialist.
Weathering steel, the third option
Weathering steel (EN 10025-5, known by the trade name Corten) is neither coated nor stainless. It is alloyed so its own rust forms a tight patina that slows further corrosion, and it stays bare. It only works where the surface wets and then dries, so permanently damp positions and salt spray zones defeat it, and run off stains the paving below. It suits fire pits and objects in garden and outdoor steel.
Colour, and why a screen is not a sample
RAL Classic provides a common colour reference, while gloss, texture, substrate and lighting still change the appearance. Compare a physical RAL reference obtained from a coating supplier with the real setting. Do not assume that the workshop supplies a sample panel: any sample, colour availability and approval method must be confirmed in the written order.
The four options side by side
| Option | Protection | Colour | When scratched | Typical use | Maintenance |
|---|---|---|---|---|---|
| Powder coating | Barrier film | Any RAL Classic | Corrosion creeps under the film | Interior steel, sheltered pieces | Wash, touch damage in early |
| Hot dip galvanising | Zinc corrodes in place of the steel | Grey only, dulls with age | Nearby zinc still protects bare steel | Gates, fencing, structures | Periodic cleaning and inspection to the project plan |
| Duplex | Zinc under a barrier film | Any RAL Classic over zinc | Zinc beneath still protects | Steelwork that must be a colour | Wash, repair damage |
| Stainless steel | Passive layer, part of the metal | Mill, brushed or coated | Repassivates itself | Coast, pool halls, handrails | Regular washing |
Quote-specific cost drivers include article mass, bath fit, vent and drain work, surface preparation, masking, colour and whether a compatible duplex system is required. Do not use generic price percentages or lifespan claims.
How to choose, in order
- Where will it stand, and how far is that from the sea, a salted road or a pool hall?
- Will it be hit? Car doors, bins, mowers and ladders decide whether you need sacrificial protection.
- Does it have to be a colour? If yes and the environment is harsh, that means duplex, not powder coating alone.
- Does the design use closed hollow sections, and are visible vent holes acceptable? If not, resolve it at drawing stage.
- Who maintains it, and how often? Choose on the assumption that nobody ever will.
Settle the finish before the drawing is fixed: it is a design decision.