Mild steel rusts. That is the trade-off for its strength and price, and the coating is what stands between a gate and its slow return to iron oxide. The three common options work in genuinely different ways, and the difference matters most where the air carries salt.
Galvanising
Hot-dip galvanising immerses steel in molten zinc, forming a metallurgically bonded zinc layer. It protects in two ways, and the second is the important one.
First, it is a physical barrier. Second – and this is what sets it apart – zinc is less noble than steel, so it corrodes preferentially. If the coating is scratched through to bare steel, the surrounding zinc sacrifices itself to protect the exposed area. The steel does not rust at the scratch.
No paint system does this. Scratch paint and rust starts immediately at that point and creeps underneath.
The trade-off is appearance: galvanising has a distinctive spangled grey finish and cannot be colour-matched without a further coating on top.
Powder Coating
Dry powder is applied electrostatically and cured in an oven, producing a hard, even, colour-consistent finish. It is tougher than conventional paint, available in any colour, and looks considerably better than galvanising.
But it is a barrier coating only. There is no sacrificial protection – once breached, moisture reaches the steel and corrosion spreads beneath the film, lifting it as it goes.
Powder coating over galvanised steel gives you both: sacrificial protection underneath, colour and finish on top. It costs more and is the right specification for exposed coastal work where appearance matters.
Paint
The cheapest option and the most dependent on preparation. A properly prepared, primed and topcoated steel item performs respectably. The same paint over mill scale or light rust fails quickly.
Paint’s real advantage is that it can be applied and reapplied on site, which makes it practical for large structures and for maintaining existing work. Its disadvantage is that it needs that maintenance on a regular cycle.
Choosing for Cape Town Conditions
Distance from the sea should drive this decision more than anything else.
Directly coastal, exposed to salt spray: galvanised, ideally with powder coating over it. Inland suburbs, general exterior use: powder coating alone performs well. Indoors or under cover: paint is usually sufficient.
Where the item is structural and replacing it would be disruptive, err towards the more durable specification. Where it is easily accessible and repainting is simple, paint is a reasonable economy.
The honest alternative worth considering: near the sea, stainless or aluminium needs no coating at all, and over the life of the item that can work out cheaper than coating and recoating mild steel.
See mild steel fabrication and coastal maintenance.
Frequently Asked Questions
Can galvanised steel be painted or powder coated?
Yes, and it is a strong combination – sacrificial zinc protection underneath with colour on top. The galvanised surface needs correct preparation first so the coating adheres properly.
Why does powder coating bubble and lift?
Usually moisture reaching the steel through a breach in the coating. Corrosion then spreads under the film and lifts it from beneath. Chips and scratches are worth touching up early for this reason.
How often does painted steel need repainting near the coast?
It depends on exposure, the paint system and preparation quality, so no single interval fits. The practical approach is inspecting annually and addressing chips and rust spots as they appear rather than waiting for a full repaint.
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