Painting galvanised steel without the paint peeling straight off

Paint lifts off galvanised steel because zinc is smooth, usually still carrying oils or a passivating treatment from the factory, and because it reacts with oil-based paint to form a soft soap layer that lets go. Degrease it, give it a key or let it weather first, prime it with a product made for galvanised metal, and the paint stays on.

The failure is almost never the topcoat. It is what went on underneath, or what was left on the zinc before anything went on at all.

Why does ordinary paint fall off zinc?

Three problems stack up.

Chemistry. Alkyd and oil-based binders react with zinc in the presence of moisture. The reaction, saponification, produces a slippery soap at the interface between the paint and the metal. The film can look perfect for a year and then come away in sheets, often releasing across a whole panel at once.

Surface. New galvanising is glassy. There is nothing for a coating to key into.

Contamination. Galvanised sheet and sections often leave the plant with quench oils or a chromate passivation treatment on them, applied deliberately to stop the zinc oxidising in storage. Both also stop paint sticking.

Is your galv new or weathered?

Establish this first, because it decides the whole approach.

Condition	How to recognise it	What it needs
New and bright	Shiny, often with a crystalline spangle pattern, water beads on it	Full degrease, then an etch primer or a primer named for new galvanised steel. The hardest state to paint
Weathered	Dull matte grey, no shine, six months or more of exposure	Wash, remove loose deposits, prime with a galvanised metal primer. The easiest state to paint
White rust	Powdery or crusty white deposits, often where sheets were stacked damp	Remove the deposit completely, wash, dry, then prime. Coating over it guarantees failure
Orange rust breaking through	Brown staining and pitting, rough to the touch	The zinc is gone locally. Treat as bare steel with a rust-inhibitive primer
Previously painted and sound	Old coating well stuck, maybe chalky	Wash off the chalk, sand to a key, spot prime bare zinc, recoat
Perforated	Light through it, flaking right through the section	Replace it. No coating repairs a hole

How do you prepare it properly?

1. Wash. Detergent and water, or a degreaser made for metal, then rinse thoroughly with clean water. This is the step people skip and the one that matters most on new galv.
2. Abrade. A non-woven abrasive pad or fine abrasive paper over the whole surface until the shine is gone. Do not use a steel wire brush or steel wool: they embed iron particles in the zinc, and those rust through your new paint as orange freckles.
3. Remove white rust. All of it. A powdery deposit is loose by definition, and nothing sticks to it.
4. Rinse and dry. Cleaning residue has to come off, and the surface has to be genuinely dry before primer.
5. Prime the same day. Freshly abraded zinc starts oxidising again immediately. In Wellington humidity, do not clean on Friday and prime on Monday.

Which primer?

One that names galvanised or non-ferrous metal on the label. That is either an etch primer, which contains an acid component that bites into the zinc, or a waterborne primer formulated with adhesion promoters for galv. Some direct-to-metal acrylics are rated for galvanised steel without a separate primer, and where they are, the datasheet says so plainly.

What not to use is a standard oil-based undercoat straight onto zinc. That is the saponification failure described above, and it is still the most common reason paint peels off spouting and garage doors.

On a large job, do a test patch. Prepare and coat a small area, wait a week, cut a cross-hatch through the film with a blade and press firm tape hard over it. If the paint comes away with the tape, the system is wrong and you have found out on a hand-sized area instead of a whole roof.

What about a galvanised roof?

Same chemistry, harder working conditions. An old galvanised roof carries lichen, moss, chalked coating and years of grime, and on the harbour edge and the south coast it carries salt as well. Salt left on the surface draws moisture through the new coating and undermines it from below, so the wash is not cosmetic, it is the job. Fasteners want checking at the same time, because a coating applied over loose or corroded screws hides a leak rather than fixing one.

Roof work also brings pitch, height and wind into it, which is a separate conversation about access and safe working. Our roof painting service covers what that involves in practice.

Spouting, downpipes, fences and garage doors

The same preparation applies to all of them, with a few practical notes.

• Spouting and downpipes are usually within reach and are a genuinely reasonable job to take on yourself, provided you degrease properly and use the right primer. They are often coated at the same time as an exterior repaint because the access is already there.
• Corrugated fences and roofing have far more surface area than their measured face suggests. Plan on using considerably more paint per square metre than a flat wall of the same dimensions.
• Sectional garage doors flex and get handled constantly. They want a flexible topcoat, and the underside of the bottom panel, where rust always starts, deserves as much attention as the face.
• Anything within reach of salt spray needs washing more often after painting, not less.

When should you leave it unpainted?

More often than you would think. Sound galvanising on a fence, a shed or a gate is already doing the job the zinc was put there for, and away from the coast it can keep doing it for decades. Painting it starts a maintenance cycle that does not stop. From that day on it is a painted surface that needs recoating, not a galvanised one that needs nothing.

The honest reasons to paint galv are appearance, matching it to the rest of a building, and extending the life of zinc that is being eaten faster than normal by salt air. On the exposed coastal side of Wellington that last reason is real. Two streets inland it usually is not, and spending the money on something else is the better call.

If you have galvanised spouting, a roof or a fence that is peeling, and you want to know whether it needs stripping back or just washing and recoating, we will take a look. Get a quote.