Spec library
Coating a galvanised substrate vs bare steel
Reference material, not engineering advice. Every technical claim traces to the cited standard or reference. Technically reviewed August 2026.
The mistake is easy to make and expensive to fix: treating hot dip galvanised steel as bare steel that has already had a coat put on it. It is not. It is a different substrate, chemically active, softer than the steel underneath, and it fails in ways bare steel does not.
Getting it wrong usually shows up the same way — a paint film that lets go cleanly, months later, taking nothing with it.
Bare steel: remove everything, then beat the clock
On uncoated steel the job is subtraction. ISO 8501-1 grades the starting condition — Grade A is fully covered with adherent mill scale and little rust; Grade D is where the mill scale has rusted away and considerable pitting is visible.
Blast cleaning removes it. Sa 2½ is near-white blast cleaning: mill scale, rust and foreign particles removed to the extent that only traces remain as slight stains. Sa 3 is white metal, everything gone.
Then the clock starts. Freshly blasted steel is chemically bare and will re-rust quickly in damp conditions. One published bridge specification requires freshly blasted steel to receive its first coating before any discolouration occurs — within 4 hours, extendable to 8 hours in a sheltered inland non-polluted environment where steel temperatures stay at least 3 °C above dew point, or as little as 1 hour in an exposed coastal environment. Its painting sections likewise require priming within 4 hours of surface preparation.
One hour, coastal. That is a scheduling constraint, not a coatings one.
Galvanised steel: you are painting zinc
Everything changes, because the surface you are bonding to is zinc, not iron — and zinc is soft, reactive, and changes character with age.
Step one: find out how old it is
The American Galvanizers Association classifies galvanised surfaces into three conditions, and they need different treatment:
- Newly galvanised — exposed to the atmosphere 48 hours or less. Minimal zinc compounds present. Needs profiling, minimal cleaning.
- Partially weathered — roughly 2 days to 1 year old. A build-up of zinc compounds plus possible organic contamination: dirt, dust, oil, grease. This is the hardest condition to prepare, and it is also the condition most steel is in when it comes back from the galvaniser and sits in a yard.
- Fully weathered — 1 year and beyond. Zinc carbonate dominates, adheres tightly, resists dissolution in water. Counter-intuitively, this needs only mild cleaning, because those compounds should stay.
Step two: clean it, and do not wait
Degrease first. Australian practice is to remove oil, grease or wax in accordance with the relevant method in AS 1627.1, then remove dirt, dust and water-soluble salts, then remove or smooth out sharp edges, dags, weld spatter and laminations to prevent uneven paint build-up.
After cleaning, allow no more than 12 hours between cleaning and coating, to limit new zinc compounds forming.
Step three: sweep blast — not blast
This is the difference that matters most. Bare steel is blasted head-on. Galvanising is swept.
- Angle: 30 to 60 degrees, not 90.
- Media: soft — particle size 200 to 500 µm, Mohs hardness of 5 or less.
- Zinc removal: Australian specifications require that no more than 10 µm of zinc be removed during brush or sweep blasting, in accordance with AS/NZS 2312.2 clause 7.5.3.2.
Ten microns is not much.
On a HDG600 coating at a nominal 85 µm, blasting away 25 µm is throwing away nearly a third of the corrosion protection you paid for — and the zinc is what is actually doing the work underneath.
Step four: prime promptly
The Australian sample specification requires primer to be applied as soon as practicable after the surface has been prepared and at least within 4 hours.
Step zero, which everyone skips: tell the galvaniser
If the article is going to be painted, the galvaniser needs to know before it goes in the bath, so that dross inclusions and skimming imperfections that would interfere with paint adhesion can be dealt with.
Why anyone bothers: the duplex effect
A duplex system — paint or powder over hot dip galvanising — does not last as long as the galvanising plus the paint. It lasts longer than the two added together.
AS/NZS 2312.2 puts the figure at 1.5 to 2.3 times the sum of the two individual lives, depending on the environment. The American Galvanizers Association quotes the same 1.5 to 2.3 range.
The mechanism is simple. The zinc is an impervious barrier under the paint, so the paint never has rusting steel underneath lifting it. And when the paint eventually goes, the zinc is still there and still working — and it needs minimal surface preparation to repaint.
Which standard governs which
| Situation | Where the requirements sit |
|---|---|
| Painting bare structural steel | AS/NZS 2312.1 (system selection), AS 1627 series (preparation), AS/NZS 3750 series (paint materials) |
| Hot dip galvanising itself | AS/NZS 4680 (the coating as manufactured), AS/NZS 2312.2 (design and durability) |
| Painting over hot dip galvanising | AS/NZS 2312.2, including clause 7.5.3.2 for sweep blasting and Table 7.1 for anti-graffiti systems; ASTM D6386 for the preparation practice |
The short version
| Bare steel | Hot dip galvanised | |
|---|---|---|
| What you are removing | Mill scale, rust, contamination | Zinc compounds, organic contamination — and as little zinc as possible |
| Preparation | Abrasive blast, typically Sa 2½, 90° | Sweep blast, 30–60°, soft media |
| Material removed | As much as needed | No more than 10 µm of zinc |
| Time to first coat | Hours — as little as 1 in an exposed coastal environment | Within 4 hours of preparation |
| Main failure mode if rushed | Flash rust under the film | Clean delamination from the zinc |
What this means for you
If your work moves between bare steel and galvanised steel — and most structural shops do — these are two different processes that happen to use the same spray gun. The preparation, the media, the angle and the clock are all different, and the one that punishes shortcuts most is the galvanised one, because the failure appears months later and takes the whole system with it.
The Paint Audit asks what kind of steel you fabricate and how you are applying, and the read comes back with your current system as you described it, alongside what your environment classification actually demands.
References
- The Australasian Corrosion Association, Steel Surface Preparation Standards
- Galvanizers Association of Australia, Sample Specification for Painting Galvanizing
- Galvanizers Association of Australia, Hot Dip Galvanizing Standards
- American Galvanizers Association, Preparing HDG for Paint
- American Galvanizers Association, Sweep Blasting Galvanized Steel
- American Galvanizers Association, Duplex Systems: Paint or Powder Coating Over HDG
- Galvanizing Association of New Zealand, An Introduction to AS/NZS 2312.2:2014
- NZ Transport Agency, S9 Specification for coating steelwork on highway structures, October 2020
- ASTM D6386, Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting
Related reading
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