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Two fence panels can both carry the word “galvanised” on their spec sheets and differ by a factor of three or more in zinc thickness — and on a coastal site that difference can decide whether the steel underneath stays protected for years or shows its first rust within a few seasons. A hot dipped galvanized fence gets its zinc after fabrication, immersed in molten zinc at around 450°C; pre-galvanised product gets a thinner zinc layer applied to the steel strip or tube before it is formed and welded. Same word, different product.
This guide explains the difference in coating thickness, how each method protects cut edges and weld points, why 42-micron-class hot-dip coatings are the coastal benchmark, and the exact specification language that separates the two when you compare quotations. It is written for buyers specifying fencing for coastal and salt-air sites across Australia, New Zealand and Canada.
Key takeaways
- Thickness gap is structural, not cosmetic: batch hot-dip galvanising typically builds 45–85 microns of zinc on standard-wall tube, while pre-galvanised tube typically carries 10–25 microns (Z100–Z275 coating classes).
- Welds are the tell: hot-dip happens after fabrication, so welds and cut lines are coated; pre-galvanised coatings are applied before forming, and welding heat disrupts the zinc at every seam.
- 42 microns is the coastal benchmark: DB Fencing hot-dip galvanises fence products to more than 42 microns, specified for coastal and harsh outdoor exposure.
- Get microns, not adjectives: a compliant quotation states the coating thickness and method in writing.
- Pre-galv has legitimate uses: budget interior runs, powder-coated products and short-term hire stock — matched to exposure, not to price alone.
Two methods, two coating builds
In batch hot-dip galvanising, the finished steel component — welded frame, panel, post — is cleaned, fluxed and immersed in molten zinc. The zinc reacts with the steel to form a metallurgically bonded coating that covers every surface the liquid touches, inside and out, including weld seams and fastener holes. Because the component is complete when it is dipped, the coating follows the product.
In pre-galvanising (also called continuous or in-line galvanising), zinc is applied to flat steel strip or tube as it is produced, before the product is fabricated. The result is a smooth, consistent, bright finish — and a much thinner coating. The steel is then cut, formed and welded into the final product, and each of those steps trades away some of the protection: cut edges expose bare steel, and welding heat burns the zinc out of the weld zone.
What the thickness numbers say
The coating thickness ranges below come from the steel industry’s own product standards and are consistent across suppliers who actually meet them:
| Method | Typical coating thickness | Reference standards | Where the coating sits |
|---|---|---|---|
| Batch hot-dip galvanising | 45–85 microns on standard-wall tube (heavier sections can exceed this) | ISO 1461, EN 10240, ASTM A123 | Applied after fabrication — welds and cuts included |
| Pre-galvanised tube/strip | 10–25 microns (Z100–Z275 classes) | EN 10346 (Z275 ≈ 19 μm per side), ASTM A653 (G90 ≈ 20 μm per side) | Applied before forming — weld zones and cut edges exposed |
Read that table as a ratio: batch hot-dip puts roughly two to four times more zinc on the steel than pre-galvanising, and it puts it where the product needs it most. Zinc protects steel sacrificially — it corrodes in preference to the steel underneath — so the service life of the coating scales broadly with its thickness. This is also why a spec sheet that says “galvanised” without a method or a micron figure is not a specification at all.
Welds and cut edges: where fences actually rust
Walk along any rusting fence line and look at where the rust starts. It is almost never the middle of a tube face — it is the weld points, the cut ends and the drilled holes. Those are exactly the locations where pre-galvanised coating is absent or damaged: the zinc was on the steel before the fabricator’s saw and welder got to it, and neither process re-applies zinc.
Batch hot-dip inverts that sequence. A welded panel goes into the zinc bath as a finished assembly and comes out with the weld seams encapsulated. Cut edges drilled on site — for signage, for bracing bolts, for gate hardware — are the one place hot-dip cannot protect after the fact, and zinc-rich touch-up paint is the standard remedy there, the same principle as repair on any hot-dip product.
For panel fencing this is not a marginal point. A temporary fence panel or a cattle panel is mostly welds — the frame is welded, the infill tubes are welded to the frame at every intersection. A coating system that fails at welds is failing across the majority of the product’s structural joints, which is why pre-galvanised panels so often show rust maps that trace their own weld patterns after relatively short coastal exposure.
Why coastal sites need the 42-micron class
Salt air accelerates zinc consumption. Chlorides are aggressive to zinc coatings, and the classic corrosion environment mapping — rural, urban, industrial, coastal/marine — puts coastal sites at the demanding end. A 10–25 micron pre-galvanised coating that lasts respectably inland can be consumed in a fraction of the time close to the surf, and the failure starts at the exposed welds and cut edges described above.
DB Fencing specifies hot-dip galvanising above 42 microns on its fence products precisely for this environment — the coating thickness is the product decision, not an upsell. Our heavy-duty temporary fence panels and livestock panels are built for the Australian and New Zealand markets, where coastal and marine-adjacent sites are a large share of demand, and the specification is written for that exposure. For buyers comparing factories, one practical test: ask what coating thickness the factory specifies for coastal orders, and note whether the answer is a number in microns or a paragraph of adjectives.
Powder coating changes the conversation slightly — a pre-galvanised tube with a good powder coat can perform well in mild exposure because the paint layer adds barrier protection — but the same rule holds: the quality of the system is decided by what happens at the welds and edges, where the coating stack is thinnest. Our article on steel fence specification for wind loads makes the same point about structure: performance lives in the joints, not the brochure.
Specification language that ends the ambiguity
Four lines in a purchase specification remove every ambiguity in a galvanising quotation:
- Method: “batch hot-dip galvanised after fabrication” — not “galvanised” alone.
- Thickness: a minimum in microns — “more than 42 microns” for coastal, with the mill or galvaniser’s certificate available.
- Verification: coating thickness is measurable on delivery with a magnetic gauge — state that measurement is part of acceptance.
- Weld protection: confirm welds are coated by the process (hot-dip) or call out the required post-weld treatment (zinc-rich paint) for any fabrication done after galvanising.
A supplier who can answer those four lines cleanly is telling you something about their process control; a supplier who answers with “don’t worry, it’s galvanised” is telling you something too. Our temporary fence panel range and round-leg corral panels both carry the hot-dip specification, and the full coating details are available in the quotation documentation.
Frequently Asked Questions
Is pre-galvanised fencing always a bad choice?
No. Pre-galvanised tube is a legitimate, economical choice for mild interior exposure, short-term use or products that receive a good powder coat system. It is the wrong choice for uncoated coastal exposure, where the thin coating and exposed weld zones corrode fastest.
How can I verify the coating thickness on delivery?
A magnetic coating-thickness gauge measures zinc thickness on steel without damaging the product, and it is standard practice for acceptance checks. Measure several points per panel, including near welds, and compare against the specified minimum.
Why do galvanised panels sometimes look dull or shiny?
Fresh batch hot-dip galvanising is typically bright and silver, weathering to a matte grey as the surface zinc reacts. Brightness varies with steel chemistry and bath conditions — it is not a reliable indicator of coating thickness, which is why measurement beats appearance.
What about scratches on hot-dip coatings during transport?
Zinc protects sacrificially, so small scratches in a thick hot-dip coating are slow to propagate rust — the surrounding zinc continues protecting the exposed steel. Deeper damage can be treated with zinc-rich touch-up paint, the standard repair for hot-dip galvanised products.
Conclusion
Hot-dip and pre-galvanising differ in thickness, in process sequence and — decisively for fencing — in what happens at the welds. Batch hot-dip coatings in the 45–85 micron class, applied after fabrication, protect the joints where fence panels actually begin to rust; pre-galvanised coatings in the 10–25 micron class cannot, because the welder and the saw came after the zinc did. On coastal sites the difference is years of service life.
Write four lines into your next specification — method, minimum thickness in microns, measurement at acceptance, weld protection — and the cheapest quotation that genuinely meets them is the right one. Everything else is comparing adjectives.
