Painting a corrugated iron fence so it lasts

A corrugated iron fence lasts if you match the primer to the metal you actually have, and if you coat the back, the bottom edge and every cut edge rather than only the side you look at. Nearly every early failure on these fences traces back to one of those two things, not to the topcoat.

One other thing worth knowing before you buy anything: if the fence is factory coated sheet that has simply gone chalky and dirty, a wash may be all it needs.

What metal is your fence actually made of?

This is the question that decides your primer, and getting it wrong is why new-looking fences peel within a year. Most corrugated fencing is either bare zinc-coated steel, which covers both galvanised and Zincalume, or factory coated sheet already in a colour. The two are treated quite differently.

What you have	How to tell	What it means for the fence
Bare metal, still new	Bright and shiny, water beads off it, no dulling yet	The hardest state to paint. Wash it thoroughly with a roof and metal wash, rinse, then use a primer made for galvanised metal. Or stand it up bare and let it weather for a season first
Bare metal, weathered	Dull grey with no shine, sometimes powdery white deposits	The easiest state to paint. Wash, remove the white deposits completely, then the same galvanised metal primer
Factory coated sheet	Already a colour, smooth baked finish	No primer while the factory coat is sound. Wash, treat any mould, sand back chalking, then a waterborne acrylic straight over it

Where red rust has broken through, the zinc has already gone at that spot, so those areas get treated as bare steel: take the loose rust back to sound metal and prime it accordingly. Which primer suits which condition of zinc, and why oil-based paint lets go of galvanising altogether, is covered properly in our piece on painting galvanised steel. On a fence, the substrate is the easy part. What follows is the part that decides whether it lasts.

The trap is new sheet. It leaves the plant slick, and paint put straight over that finish is stuck to the finish rather than to the metal, so it comes away in sheets a year later. Washing it properly is not optional, and the data sheet for the primer you have bought will tell you whether it also wants the surface keyed first. Follow that sheet rather than a rule of thumb, because primers made for galvanised metal differ from one another on exactly this point.

Which part fails first?

The bottom edge and the back. The face you painted gets rinsed by every shower of rain and dries in the wind. The back sits against the neighbour's hedge or a shaded garden bed, never dries properly, and often never got a coat in the first place. The bottom edge sits in soil, bark chip or wet grass, which is a permanent damp poultice held against steel.

If the fence is not up yet, coat both faces on the ground. It costs an hour and it is the single biggest thing you can do for how long the fence lasts. If it is already standing and you cannot reach the back, at least clear the base: pull soil and mulch back from the sheet, cut planting away so air can move, and make sure the sheet is not sitting in a garden bed.

What about cut edges and screws?

A cut edge has no zinc on it. Galvanising is applied to the sheet, so every time someone runs a grinder or nibbler through it, the steel exposed at that cut is unprotected. Cut edges along the top of a fence, around a gate and at the end of each run need priming deliberately, not just whatever the roller happened to reach.

Fixings matter more than people expect. A rusting screw head bleeds a brown streak down the corrugation below it, and a fence full of those streaks reads as finished even when the sheet itself is sound. Use fasteners suited to the sheet, avoid putting incompatible metals in contact with each other, and spot prime any fixing already showing rust before the topcoat goes anywhere near it.

Does Wellington change anything?

Two things, and both are worth planning around.

Wind. A fence is a solid sail, and this city tests that constantly. Movement at the fixings works the coating loose exactly where the screw meets the sheet, which is the last place you want a break in the film. A fence that flexes noticeably in a good northerly has a fixing or a post problem, and coating it before sorting that out is money in the wrong place.

Salt. Anywhere near the south coast or the harbour edge, fences corrode noticeably faster than they do a few kilometres inland. A fresh water rinse twice a year, including the sheltered side that rain never reaches, does more for the life of the coating than any upgrade in product. That is the same logic that applies to the rest of a coastal house.

Brush, roller or spray?

Corrugations make this a real question. A roller rides over the tops and misses the valleys, so a roller-only job is half a job. A brush reaches the valleys but is slow across a long boundary. Spraying is the fastest and gives the most even film, and it is also how you paint the neighbour's car.

The practical answer for most boundary fences here is a roller on the flats with a brush cutting into the valleys and around every fixing. If you do spray, do it on a genuinely still morning, mask what is behind you, and talk to the neighbour first. Still mornings are the limiting factor in this city, not skill.

What colour should it be?

Mid to light colours are easier on the coating. Dark colours on metal in full sun run hot, and that heat cycling puts more stress on the film and the fixings than the same colour would on a timber fence. If you want dark, it is not a disaster, but expect a shorter interval before it starts looking tired.

The other consideration is not technical. A boundary fence is usually a shared asset, and a conversation before you start is a great deal cheaper than the alternative. It occasionally gets you half the paint bill as well.

When should you not bother?

• If the sheet is perforating, meaning actual holes rather than surface rust, coating buys very little. Replace those sheets, then coat.
• If it is factory coated and only looks tired, wash it first and look again. Chalky baked finishes come back a long way with a wash and a soft brush.
• If the posts are moving, fix the posts. A coating cannot hold a fence up.
• If the fence is new galvanised and you are happy with a weathered grey look, leave it. Galvanising is a coating in its own right and will last for years untouched. Painting it is a preference, not a necessity.

This is also one of the better do-it-yourself jobs on a property. It is at ground level, the surface is forgiving, and mistakes are easy to correct. The skill is entirely in the preparation, and the preparation is washing and priming, which anyone can do carefully.

If the fence is part of a larger exterior job, or the metalwork on the house needs the same treatment, our exterior painting work covers both. Tell us what you have and we will say whether it is worth a coat or a replacement.