Paint peeling off galvanised steel in sheets is one of the most common and most avoidable coating failures. The zinc surface presents three obstacles, and each has a specific answer.
Obstacle one: passivation treatments
Newly galvanised steel is often treated with a chromate or similar passivation to prevent wet storage staining during transport. That layer is designed to stop things reacting with the zinc, including paint. It must be removed by sweep blasting or appropriate chemical treatment before coating, or the paint bonds to a layer that itself detaches.
Obstacle two: alkalinity and saponification
Zinc surfaces are alkaline in the presence of moisture. Alkyd based paints saponify in those conditions, meaning the binder chemically breaks down at the interface and adhesion is lost. This is why alkyd enamels should not be applied directly to galvanising regardless of how well the surface was cleaned, a point noted in our enamel versus epoxy article.
Obstacle three: a smooth, non porous surface
Fresh galvanising is smooth and offers little mechanical key. Weathered galvanising develops zinc corrosion products that provide better tooth but also require cleaning, since loose white rust is not a sound base.
The workable approaches
Sweep blasting. A light abrasive pass at low pressure with fine abrasive removes passivation and creates profile without stripping the zinc layer. This is the most reliable preparation, and pressure control matters because excessive blasting removes the galvanising you paid for.
Dedicated primers. Etch primers and adhesion promoting primers formulated specifically for zinc surfaces bond chemically where ordinary primers cannot. These are the practical route where blasting is impossible on site.
Weathering. Allowing galvanising to weather for several months lets passivation break down naturally and develops surface texture. Effective but rarely compatible with a construction programme, and the weathered surface still needs washing to remove loose corrosion products and salts.
Confirming before committing
Where the galvanising history is unknown, apply a test patch of the intended system, allow full cure and carry out a cross hatch adhesion test as described in our adhesion testing article. Two days of testing avoids stripping an entire structure.
MENA Industrial Supplies supplies primers suitable for galvanised and non ferrous substrates with data sheets stating preparation requirements.