Untreated steel will rust when it is exposed to moisture, so most structural steel is supplied with some form of protection. The two finishes we are asked about most are primer and hot-dip galvanising. Which one you need depends mainly on where the steel will live.
Bare (black) steel
Steel straight from the mill has a dark mill-scale surface. It is fine for steel that will be fully encased, or that someone else will prepare and paint, but it will start to show surface rust quickly if it is stored outside.
Primed steel
A coat of primer gives the steel temporary protection and a good base for decorating. It suits steel that stays inside a dry, heated building, such as a beam in a knocked-through wall or a loft conversion. Primer on its own is not a long-term outdoor finish: steel that will be exposed should get a suitable top coat or a more durable system.
Hot-dip galvanised steel
Galvanising dips the finished steel into molten zinc, which bonds to the surface and protects it from corrosion, including inside hollow sections where the zinc can reach. It is the usual choice for steel outside or in damp conditions: balconies, external posts, canopies, gates, frames and anything near the ground or the coast. In the UK it is normally carried out to BS EN ISO 1461.
One practical point: cutting, drilling and welding should be done before galvanising, so that the cut edges and holes are protected too. Plan your fabrication first.
Quick guide
- Inside, dry, will be boxed in or decorated: primed is usually enough.
- Outside, damp or hard to maintain: galvanised is the safer choice.
- Specified on the drawing or by building control: always follow the specification.
From our yard
See our finishes page for the options we offer. We can cut and drill to your drawing before the steel goes for finishing.
This guide is general information only. The size and grade of any structural member must be specified by a qualified structural engineer or building control-approved design.
Need a price on primed or galvanised steel? Send us your drawings or get in touch, and we’ll quote the steel and finish together with a delivery date that suits you.