Short answer: for outdoor steel, hot-dip galvanising to AS/NZS 4680 is the better protective finish, because the zinc is metallurgically bonded to the steel, coats inside hollow sections and protects small damaged areas sacrificially. Powder coat is a colour and appearance finish. On exterior work it belongs over galvanising (a duplex system), not directly on bare steel. Indoors and dry, pre-galvanised or primed steel is usually enough.
Every finish question is really a lifespan question. Bare steel rusts; the only choice is which barrier you put in the way and how long it has to last before anyone touches it again. This guide sets out how the main finishes differ, what the Australian standards say, and how we choose a finish product by product.
What is the difference between hot-dip galvanising and powder coating?
Hot-dip galvanising immerses the finished, cleaned steel in molten zinc, which reacts with the steel to form zinc–iron alloy layers under a zinc outer layer. Powder coating sprays a dry polymer powder onto the surface and bakes it into a continuous film. One is a bonded metal coating; the other is a paint-type barrier.
That difference drives everything else. A galvanised coating reaches every surface the zinc can flow to, including the inside of a hollow section once vent and drain holes are provided. If it is scratched, the surrounding zinc corrodes in preference to the exposed steel, so small damage does not creep. A powder coat only protects while the film is intact. Once it is chipped or cut, moisture reaches the steel and rust can spread underneath the coating. What powder coat does well is colour, gloss and a smooth, cleanable surface, which is why it is often specified over galvanising rather than instead of it.
How long does hot-dip galvanising last in Sydney conditions?
It depends on how thick the zinc is and how corrosive the site is. Coating life is roughly proportional to zinc thickness, and zinc is consumed faster near breaking surf and in industrial atmospheres than in dry inland suburbs. There is no single number; you estimate life for the specific site.
Australian practice classifies sites into atmospheric corrosivity categories, from C1 (very low) to C5 and CX (extreme), using AS 4312 Atmospheric corrosivity zones in Australia, which aligns with ISO 9223. AS/NZS 2312.2 then gives guidance on the durability of hot-dip galvanised coatings in each category. The Galvanizers Association of Australia (GAA) publishes a free durability of galvanizing estimator that works through the same inputs: coating thickness, site location and exposure. Use it, or ask your engineer, for a site-specific figure rather than relying on a rule of thumb.
In practical terms, most of suburban Western Sydney sits in the lower categories, while beachfront and harbour-edge sites move up the scale. The closer the steel is to breaking surf, the more the coating thickness and the choice of duplex or stainless matter.
How thick is a hot-dip galvanised coating?
AS/NZS 4680 sets a minimum coating thickness that increases with the thickness of the steel being galvanised, because thicker steel holds heat longer and grows a thicker alloy layer. Coating thickness is checked on the finished article, usually with a magnetic gauge, against both a local minimum and an average minimum.
| Steel thickness | Local minimum (µm) | Average minimum (µm) | Average minimum mass (g/m²) |
|---|---|---|---|
| ≤ 1.5 mm | 35 | 45 | 320 |
| > 1.5 mm to ≤ 3 mm | 45 | 55 | 390 |
| > 3 mm to ≤ 6 mm | 55 | 70 | 500 |
| > 6 mm | 70 | 85 | 600 |
Minimums for articles that are not centrifuged, as tabulated in AS/NZS 4680. Check the current edition through Standards Australia before quoting these in a specification.
Two things follow from the table. A 4 mm wall SHS pole and a 10 mm base plate welded to it will carry different coating thicknesses on the same part. And galvanised coatings are often thicker than the minimum, particularly on steels with higher silicon content, which is normal and does not reduce performance. The GAA’s specification and inspection guidance covers how coatings are measured and what surface appearance is acceptable.
Is powder coat on its own enough for outdoor steel?
Usually not for structural or long-life outdoor work. Powder coat on bare steel is a barrier with no back-up, so any chip, cut edge or damaged corner becomes a starting point for rust that travels under the film. It suits indoor items, light-duty outdoor items that are easy to recoat, and anything already galvanised underneath.
Powder coating for architectural work is covered by AS 4506 Metal finishing — Thermoset powder coatings. Its performance depends heavily on surface preparation and pre-treatment. Edges and welds are the weak points, because the film is thinnest there and they are where knocks land. If a colour is required outdoors, we recommend putting the powder over galvanising rather than accepting a maintenance cycle on painted bare steel.
What is a duplex coating, and when is it worth it?
A duplex system is hot-dip galvanising with a paint or powder coat applied over the top. The zinc protects the steel; the organic coating protects the zinc and supplies the colour. It is worth it when steel must match a colour, sits front-of-house, or is in a more corrosive site where you want the longest time to first maintenance.
The two layers help each other: the topcoat slows the rate at which the zinc is consumed, and the zinc stops rust creeping under the topcoat when it is damaged. The GAA and international galvanizing associations describe the combined life as longer than the two coatings would give separately. The catch is preparation. Powder coating over galvanising needs the right pre-treatment, and hot-dip galvanised surfaces can outgas during curing and cause pinholes unless the coater manages it. Use a coater who does duplex work routinely, and specify the system rather than just a colour.
On our range, this is why the horizontal mounting bracket and the 100×100×4 SHS equipment mounting pole are offered either hot-dip galvanised or powder-coated safety yellow. Galvanised is the long-life choice; the powder-coated colour is for high-visibility sites where hazard marking is part of the specification.
When is pre-galvanised or primed steel good enough?
Pre-galvanised and primed steel suit dry indoor work and jobs where a painter will finish the steel on site. Pre-galvanised hollow sections are zinc coated in-line at the mill under AS/NZS 4792, with a much lighter zinc coating than batch hot-dip galvanising, so they are not a like-for-like substitute outdoors.
With pre-galvanised steel, every cut and weld made after galvanising is bare steel and needs a zinc-rich touch-up. Shop primer is a temporary protective coat and a base for a site-applied paint system under AS/NZS 2312.1; left as the only finish outdoors, it will not last. On our PFC channels, the choice is presented as hot-dip galvanised or primed. On that product page we note that galvanising adds roughly 10–15% over the primed price, which is small against the cost of repainting steel in place.
Which finish should I choose for my project?
Start from the environment and the design life, then add appearance. Exterior structural steel is galvanised by default. Colour-matched or front-of-house steel is duplex. Coastal and marine sites are duplex or stainless. Dry interiors can be pre-galvanised or primed. Feature landscaping may use Corten with no coating at all.
At a glance
| Finish | Best use | Protects inside hollow sections? | Damage tolerance | Maintenance outlook |
|---|---|---|---|---|
| Hot-dip galvanised (AS/NZS 4680) | Exterior structural, poles, lintels, civil edging | Yes, with vent and drain holes | High: zinc protects small scratches | Site-dependent; estimate with GAA tools |
| Powder coat on bare steel (AS 4506) | Indoor items, easily recoated light-duty items | No | Low: rust creeps from chips | Recoating cycle |
| Duplex (galvanise + powder or paint) | Colour-matched, front-of-house, more corrosive sites | Zinc inside; topcoat outside | High | Longest to first maintenance |
| Pre-galvanised (AS/NZS 4792) or primed | Dry interiors; steel painted on site | Pre-galv: yes, light coating; primer: no | Low at cuts and welds | Touch-up or site paint system |
| Stainless (304 / 316) | Coastal edges, pool surrounds, food areas | Not applicable: solid alloy | High | Wash-down; pickle and passivate welds |
| Corten weathering steel | Feature landscaping | Not applicable | Self-forming patina | None, but plan for run-off |
For civil work, the cast-in angle edging and our steel packers are supplied hot-dip galvanised because they are hard or impossible to reach once they are cast in or grouted. For marine edges, see ASSDA’s guidance on when to step up to 316 stainless, and our guide to Corten and architectural garden steel.
Key takeaways
- Hot-dip galvanising to AS/NZS 4680 is the default for exterior steel. It is bonded, coats inside hollow sections and protects scratches sacrificially.
- Coating life depends on zinc thickness and the site’s corrosivity category (AS 4312 / ISO 9223). Estimate it per site; do not assume a single figure.
- Powder coat on bare steel outdoors becomes a maintenance commitment. Put it over galvanising (duplex) when you need colour.
- Pre-galvanised and primed steel suit dry interiors or site-painted work. Treat every cut and weld.
- Stainless and Corten are materials, not coatings. Choose them for the environment and the look.
Not sure which finish your site needs? Tell us where the steel is going (suburb or distance from the coast), whether it needs a colour, and how long it has to last before first maintenance. We will recommend galvanised, duplex or stainless and confirm the price on your quote request.
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