TECHNICAL 05 · COATING

Pre-galvanised coil or hot-dip after fabrication: the choice that decides where rust starts.

Two methods, two different protection patterns. Pre-galvanised protects the surface it was rolled with and leaves every cut end bare. Hot-dip coats the finished part, inside and out, including welds and cut ends.

Round tube bundles, stacked in the mill — XinYonSen

The two methods side by side
	Pre-galvanised (Z275 coil)	Hot-dip (ISO 1461)
When coating happens	At the steel mill, on coil	After fabrication, on the finished part
Cut ends	Bare steel	Coated
Welds	Bare, unless touched in	Coated
Coating thickness	≈ 20 μm per side (275 g/m² total)	Typically 45–85 μm, class-dependent
Inside of tube	Coated as rolled	Coated if vented for dipping
Surface	Smooth, uniform	Slightly rougher, drain marks possible
Typical use	Roll-formed purlins, gutters, arches from coil	Welded frames, columns, humid sites
Cost	Lower	Higher, plus handling

Z275 means 275 g/m² total zinc across both surfaces; that is about 137.5 g/m² per side, roughly 19–20 μm.

Where pre-galvanised fails first

A cut end: no zinc at all, and the wall is exposed.
A punched hole: the punch drags coating away at the edge.
A weld: the heat destroys the coating for a zone around it.
A rub point in a bundle: coating worn off in transit.

All four are fixable — touch-in with a zinc-rich coating, edge protection in packing, and a written requirement that field cuts be touched in. What is not fixable is a buyer discovering it two years later.

Where hot-dip is the right answer

Humid climates, coastal sites, livestock buildings with ammonia, glasshouses where the frame must last decades, and any project where the inspector will check coating thickness at a weld. In those cases the extra cost is a fraction of the frame and it removes an argument.

One limitation of hot-dip

Long tube must be vented so the zinc can flow and the air can escape; sealed hollow sections cannot be dipped properly. If a design needs sealed sections, say so early — the detail changes before fabrication, not after.

A specification sentence that works

“All steel to be hot-dip galvanised to ISO 1461 after fabrication, minimum coating 55 μm, with venting and draining holes as required; cut ends and field cuts to be touched in with zinc-rich coating supplied with the order.” That is a sentence an inspector can test.

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