Technical guide
Both protect steel; they fail differently. Galvanising is a metallurgical barrier that sacrifices itself, paint is a physical barrier that must stay intact. The right choice is driven by access for maintenance, not by the price per square metre.
| Hot-dip galvanising (EN ISO 1461) | Paint system | |
|---|---|---|
| Mechanism | Zinc-iron alloy layers, sacrificial | Barrier film, no sacrificial action |
| Typical thickness | 55–85 µm (≈ 395–610 g/m²) | 120–240 µm DFT for C3–C4 |
| Life, rural/urban | 25–40 years | 10–15 years before first maintenance |
| Damage behaviour | Small scratches self-heal by zinc sacrifice | Scratch becomes a corrosion start point |
| Appearance | Grey, slightly rough, spangled | Any RAL colour, smooth |
| Cost indicator | Per tonne of steel, roughly 1,800–2,600 CNY/t in China | Per m², strongly dependent on system and colour |
Detailing rules that decide whether the coating survives: vent holes at both ends of every sealed section, drain holes at the low point, no overlapping surfaces that trap pickling acid, and a minimum wall of about 3 mm for long hollow sections so the 450 °C bath does not distort them.
Best practice for exposed structures is the duplex system: galvanise first, then paint. The paint slows zinc consumption and the zinc protects the paint edges; life is typically 1.5–2.5× the sum of the two alone. It costs more upfront and is usually the cheapest option over 30 years.
hot-dip galvanised to EN ISO 1461, 55–85 µm, vent and drain holes per drawingshot blast Sa 2.5, epoxy primer 80 µm + polyurethane topcoat 60 µm, RAL 7016We quote the coating as a separate line so you can see what it costs against the steel. Related: 100, 150, 200.
Yes, roughly 3–6 % on thin sections (zinc is about 7.1 g/cm³). A 55 µm coating adds around 0.39 kg/m².
Welding burns the zinc and produces fumes — always weld first, then galvanise. If you must weld after, grind back 50 mm and repair with zinc-rich paint.
White rust (zinc oxide) forms when newly galvanised steel is stacked wet. Specify ventilation during transport and dry storage on site.