AS/NZS 2312 C5 Marine Paint Spec for NZ Coastal Steel — duplex vs galv-only

sp8002
2 hours ago
7 min read

Hot-dip galv is the default paint spec for NZ structural steel — and it's right most of the time. On a Waiheke, Northland, or exposed Wellington site it's not enough. AS/NZS 2312 halves the warranty term in C5 unless you upgrade to duplex. On a $90-130k steel package that's a $5-15k decision worth getting right at tender.

By Steve Parker · Trueworks · NZ construction estimation · 6 min

What you'll learn in this post

What AS/NZS 2312 actually says
The duplex system, in plain English
Where the spec hides in the documents

Quick answer: AS/NZS 2312 classifies NZ atmospheric corrosivity from C1 (interior dry) to C5 (severe marine). Hot-dip galv to AS/NZS 4680 delivers 30-50 years to first maintenance in C3 but only 15-25 years in C5. The standard C5 upgrade is a duplex system — galv + epoxy primer + polyurethane topcoat, total DFT 235-325 µm — which delivers 25+ years in C5. On a typical $90-130k residential structural steel package, the duplex upgrade adds $5-15k and the warranty consequences of getting it wrong run into the $20-40k range over the building's life.

Hot-dip galvanising to AS/NZS 4680 is the default coating system for fabricated NZ structural steel. On most residential and light-commercial work it's the right answer — a 75-100 µm zinc coating, properly inspected, gives 30-50 years of corrosion protection in C3 (medium / moderate) atmospheric conditions. That covers most of central Auckland, most of inland NZ, and most of NZ's commercial-suburban building stock.

What galv-only doesn't cover is C5 — severe marine atmosphere. AS/NZS 2312 (the standard governing protective coatings for structural steel) zones NZ into corrosivity categories C1 (interior dry) through C5 (severe marine / industrial). In C5, hot-dip galv on its own halves the practical-life expectancy. The standard upgrade is a duplex coating system — galv plus an epoxy primer plus a polyurethane topcoat.

On a typical residential structural steel package ($90-130k for 7-10 tonnes), the cost of upgrading from galv-only to duplex is typically $5,000-15,000. The cost of not upgrading is a 15-25-year first-maintenance interval instead of 30-50 years, and a contestable warranty position from year 10 onwards.

What AS/NZS 2312 actually says

AS/NZS 2312 is a two-part standard:

AS/NZS 2312.1 — coatings using protective paint systems
AS/NZS 2312.2 — coatings using hot-dip galvanising

Both parts reference the same atmospheric corrosivity categories from ISO 9223, which AS/NZS 2312 has adapted for the Australasian context:

| Category | Description | Where in NZ | |---|---|---| | C1 | Very low — heated interior, controlled humidity | Inside conditioned buildings | | C2 | Low — dry interior, occasional condensation | Most interior framing under cladding | | C3 | Medium — urban / industrial with moderate pollution | Most of inland Auckland, Hamilton, urban Christchurch | | C4 | High — industrial, coastal areas with moderate salt-spray exposure | Coastal Auckland (>200m from coast), light industrial | | C5 | Very high (marine) — coastal areas with high salinity | Waiheke, Northland coast, exposed Wellington, west coast | | CX | Extreme — offshore, industrial chemical | Offshore platforms (rare in NZ residential) |

The category drives the coating system. For a steel member in C5 atmosphere, AS/NZS 2312.1 Table 6.2 (or the engineer's specific paint spec) names a coating system that delivers 25+ years to first maintenance. Galv-only — AS/NZS 2312.2 — delivers that long-life in C3 but not in C5.

Send us your stamped drawings + the supplier's quote. We'll return a code-cited Quote-Check packet inside 5 business days. Free for first-time customers. NDA available, NZ-hosted processing.
→ Email steve@trueworks.co.nz or submit at trueworks.co.nz

💡 Caught something similar on your job?

The duplex system, in plain English

A duplex coating on structural steel typically reads:

| Layer | Spec | Typical DFT | |---|---|---| | 1 | Hot-dip galvanise to AS/NZS 4680 | 85-100 µm zinc minimum | | 2 | Surface preparation of galv (sweep blast, T-wash, or etch primer) | — | | 3 | Epoxy primer (industrial-grade two-pack epoxy) | 100-150 µm DFT | | 4 | Polyurethane topcoat (industrial-grade two-pack polyurethane) | 50-75 µm DFT |

Total DFT 235-325 µm, with a service life in C5 of 25+ years to first maintenance.

The galv provides the cathodic protection (zinc oxidises before steel does). The epoxy provides the chemical barrier. The polyurethane provides the UV-stability and the colour. Each layer does a different job. Skip any layer and the system doesn't deliver the rated life.

The Trueworks Per-Project pack is NZ$8,000–12,000 for the full estimation, risk register, contingencies table and pre-trade-start sheets across all major trades on a build. Builders typically save NZ$10,000–20,000 of their own time per job.
See pricing on trueworks.co.nz →

📋 Want this kind of risk review on every trade?

Where the spec hides in the documents

On a typical NZ residential structural set, the paint spec lives in one of four places — and rarely in all four:

The architectural specification's paint section (sometimes detailed, often generic "paint per engineer's spec")
The structural calcs Appendix B or "Corrosion Protection" section (where most engineers put the actual spec — worth finding)
The structural drawings general notes sheet (often a one-liner naming AS/NZS 4680 only — the right answer for C3 and the wrong answer for C5)
The specifier's brief or basis-of-design memo

A fabricator pricing the work reads the structural drawings. If the C5 duplex spec isn't there, it doesn't get priced — and the variation lands at month 4 when the engineer issues an RFI.

The four members that always need the C5 check on a coastal build

On a typical coastal residential, four member types sit in C5 atmosphere and need the duplex spec called up explicitly:

Entry canopy framing — exposed on three or four sides. Often PFC or RHS. Direct salt-spray.
Veranda / pergola posts and beams — same exposure as the canopy. Often SHS or UC on post-base brackets.
Louvre beams and balcony framing — visible architectural elements where duplex is also a finish requirement.
Roof-edge structural members and outrigger steel — cantilevered projections, eaves, sun-shading. Direct rain wash.

Internal steel behind the cladding line generally remains galv-only or mill-finish.

Worked example — a typical 7-10 tonne residential coastal steel package

From a recent coastal residential build, 7-10 tonnes of structural steel, $90-130k headline supplier quote. The supplier's quote priced the galv but listed the paint section as "internal red oxide shop primer only." Reading the engineer's structural calcs Appendix B revealed C5 duplex required for the exterior-exposed members (entry canopy, pergola beams, louvre beams — about 30-35% of the total tonnage).

| Stage caught | Cost | |---|---| | At tender stage (duplex upgrade as a line item) | $4,000-6,000 | | At month 4 (engineer RFI, strip-back, re-coat, programme delay) | $9,000-14,000 |

Caught at tender = $5k line item. Caught at month 4 = $9-14k all-in plus programme cost.

The warranty implications

Coating-system warranties from major NZ industrial coatings suppliers are explicit about corrosivity zoning. A typical 25-year coating warranty issued for a duplex system in C5 reads in summary:

"Warranty applies to coating system as specified, applied to AS/NZS 2312 and AS/NZS 4680 standards, on substrate prepared to ISO 8501-1 Sa 2½, in atmospheric category C5 marine. Annual visual inspection and biennial wash-down required. Warranty excludes mechanical damage, lap-joint failures, and surfaces continuously immersed."

The annual inspection and biennial wash-down clauses are the ones owners forget. On a coastal residential, a $300/year wash-down is the difference between a 25-year warranty in force and a contestable position at year 8.

What to check on every coastal NZ steel quote

Seven items on a Waiheke / Northland / exposed Wellington / west coast build:

Atmospheric category named — C3, C4, or C5 — with reference to the engineer's assessment or AS/NZS 2312 Annex E
Coating system named in the quote with actual products and DFTs, not just "as per engineer"
Member-by-member spec — which members are duplex, which are galv-only, which are internal-only
Inspection and warranty conditions — annual wash-down, inspection intervals, who provides them

Structural calcs Appendix B reference confirming the engineer's coating spec matches the supplier's quote
Fastener spec to AS/NZS 3566 Class 5 or stainless — bolts and screws have to match the coating spec
Touch-up procedure for site damage, with the touch-up product named and the certified applicator identified

FAQ — AS/NZS 2312 C5 marine coating for NZ structural steel

Q1: Is duplex coating required for all coastal NZ residential steel, or only directly-exposed members? AS/NZS 2312 categorises by atmospheric exposure, not building element. Internal steel behind cladding in a coastal building is typically C2 or C3 and stays galv-only. Steel directly exposed to the marine atmosphere (entry canopy, pergola, louvre, outrigger) is C5 and needs duplex. The engineer's coating spec names which members are which.

Q2: What's the warranty difference between galv-only and duplex in C5 atmosphere? Galv-only in C5: typically 15-25 years to first maintenance (AS/NZS 2312.2 service life). Duplex in C5: typically 25+ years to first maintenance (AS/NZS 2312.1 service life for the named system). The 10-15 year delta is the warranty-stakes difference.

Q3: Does the duplex coating need to be applied before or after fabrication welding? Galv is applied after fabrication is complete. Field welds on galvanised steel require touch-up with a cold-galv repair product to restore the zinc coating; the epoxy and polyurethane are then applied over the touched-up area. Touch-up procedure must be named in the supplier's quote.

Q4: How much more expensive is duplex vs galv-only per tonne of steel? Typically $600-1,500 per tonne of steel for the duplex upgrade, depending on member geometry (surface area per tonne) and the specific product range. On a 7-10 tonne residential package, that's $4-15k total, but only on the 30-40% of tonnage that's in C5 exposure.

Q5: Are there C5-marine alternatives to duplex that NZ engineers commonly specify? Yes — thermal metal spray (TSA), 3-coat zinc-epoxy-polyurethane without galv (where galv kettle access is limited), and proprietary high-performance coating systems from specialty industrial-coatings suppliers. The engineer's coating spec is the authoritative answer; AS/NZS 2312.1 Table 6.2 sets the durability benchmark each alternative has to meet.

Get a free Quote-Check on your next job

Drawings + one supplier quote + 5 business days = a code-cited risk packet ready for your pre-start meeting.

Free for first-time customers. No commitment. NDA available. Files NZ-hosted, deleted after 30 days unless you ask us to retain them.

→ Email steve@trueworks.co.nz or start the intake at trueworks.co.nz

About Trueworks

Trueworks is built by Steve Parker — 20 years on the analytical side of NZ construction. Variation reviews, contract advisory, programme review, and AI-augmented document workflows. Trueworks is the productisation of that practice for builders: same defensible analysis, at a price and pace a NZ builder can actually use.

I answer every email personally during pilot phase. If you've got a quote you want a second opinion on, the easiest way to find out if Trueworks is useful is to send it.

📧 steve@trueworks.co.nz · 🌐 trueworks.co.nz