Duplex systems: painting galvanized steel without losing adhesion

To paint galvanized steel, first clean oils and salts off the zinc, remove drips and white corrosion products, then create a light profile by brush-off blasting per SSPC-SP 16 or another preparation allowed by ASTM D6386, before applying a primer designed for zinc, usually epoxy or acrylic. Alkyd must not go directly on zinc: it saponifies and peels.

A duplex system combines hot-dip galvanizing with a paint system. The paint protects the zinc from the environment, and the zinc protects the steel where the paint is damaged. Together they last longer than either one separately, which explains their use on bridges, structures in aggressive environments and visible equipment. The weak point is paint adhesion to zinc. This guide is for maintenance managers and project managers in Quebec and Ontario. It complements our steel structure painting page.

Why combine galvanizing and paint?

Galvanizing and paint are combined because each makes up for the other's weakness. Zinc is consumed slowly but steadily in aggressive atmospheres; paint eventually develops defects. Under paint, zinc is consumed far more slowly, and where the paint is damaged, the zinc keeps the defect from turning into red rust.

The duplex system is also chosen for practical reasons:

• Colour and marking. Galvanizing alone is grey; the topcoat gives the required colour.
• Severe environments. Industrial or marine atmosphere, de-icing salts, splash.
• Deferred maintenance. Repainting is done before the zinc is reached, on a surface that is still sound.

Hot-dip galvanizing is done per ASTM A123, CSA G164 or ISO 1461, and ISO 12944-5 describes paint systems on galvanized steel. Our comparison of cold galvanizing vs hot-dip places the duplex system among the zinc protection options.

Why does paint adhere poorly to zinc?

Paint adheres poorly to zinc when the surface is too smooth, contaminated or chemically reactive. New zinc is smooth and sometimes passivated as it leaves the bath; weathered zinc carries white corrosion products and salts that come off with the paint.

There are three surface conditions:

• New galvanizing. Smooth, bright surface, often quenched or passivated, which hinders adhesion.
• Partially weathered galvanizing. A mix of metallic zinc and oxides, sometimes with white rust from wet storage.
• Fully weathered galvanizing. A stable layer of corrosion products, easier to paint if sound and clean.

Each condition calls for a different preparation, which ASTM D6386 sets out.

To find out which condition you are dealing with, first ask the galvanizer about any post-treatment applied as the steel left the bath, then examine the surface: gloss, white spots, rough areas. On an existing structure, an adhesion test on a small area prepared with the planned method confirms the choice before the job starts. This check takes little time and avoids discovering widespread peeling after the topcoat is on. It also gives the owner a reference point for later maintenance inspections.

How should zinc be prepared before painting?

Zinc is prepared in three steps: removing contaminants, removing irregularities and corrosion products, then creating a light profile or a reactive surface depending on the chosen system. The aim is to anchor the paint without removing a significant thickness of zinc.

• Cleaning. Removal of oil, grease and salts per SSPC-SP 1, then rinsing with clean water.
• Irregularities. Drips, runs and zinc spikes dressed by hand or power tool, without exposing the steel.
• White rust. Zinc corrosion products removed by non-metallic brushing or light sweep blasting.
• Profile. Light brush-off blasting per SSPC-SP 16, with a fine, non-metallic abrasive and reduced pressure, until the surface is uniformly matte.

Brush-off blasting is not full blast cleaning: it does not aim for a steel cleanliness grade, but for a profile on the zinc. Our abrasive blasting page describes the appropriate settings. For powder coating over galvanized steel, ASTM D7803 gives the corresponding preparation.

Which primers are compatible with zinc?

Primers compatible with zinc are those the manufacturer specifically intends for galvanized steel, most often epoxies or acrylics, sometimes polyurethane systems. The data sheet states the required surface condition and preparation.

The table below summarizes the families and their usual use; the specification and the data sheet decide.

The complete system (primer, intermediate, topcoat) is chosen for the corrosivity category, as our guide to ISO 12944 C3, C4 and C5 paint systems shows. Thicknesses are recorded in a dry film thickness log.

Which mistakes make a duplex system peel?

Most peeling comes from an incompatible primer or a skipped preparation step, not from the galvanizing itself. The defect often appears quickly, in whole sheets that leave the zinc bare.

• Alkyd directly on zinc. Zinc reacts with the oils in the alkyd and forms soaps at the interface: the paint loses adhesion.
• Unprepared new galvanizing. A smooth or passivated surface does not hold the primer.
• White rust left in place. The paint comes off with the corrosion products.
• Overly aggressive sweep blasting. Too coarse an abrasive or too high a pressure removes the zinc and exposes the steel.
• Too long after preparation. The zinc reoxidizes or is recontaminated before priming.

When a galvanized structure has already been painted, our article on painting over old coatings helps decide between overcoating and removal.

How to maintain a duplex system?

A duplex system is maintained by monitoring the paint and renewing it before the zinc is reached. As long as the zinc is intact under the paint, maintenance repainting is enough; where steel is exposed, the zinc has to be repaired first.

• Periodic inspection. Peeling, chalking, scratches, areas of white or red rust.
• Paint touch-up. Cleaning, local preparation, compatible primer and topcoat.
• Zinc repair. Where steel is exposed, preparation then repair per ASTM A780, often with a zinc-rich coating. Our cold galvanizing page describes these repairs.
• Full repainting. Planned when the paint is worn over large areas, before the zinc is reached.

Our article on maintenance painting of steel structures details the approach.

How to get a quote for a duplex system?

You get a suitable quote by stating whether the galvanizing is new or weathered, whether it has already been painted, the service environment, the required colour and access constraints. These details decide the preparation and the system.

Duplex systems are part of our protective coatings work. Our crews and qualified subcontractors work from LaSalle, including Montérégie and Hamilton. Send photos, dimensions and a description of the structure through the request a quote form: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Should galvanizing be left to weather before painting?

No. New galvanizing can be painted provided it is prepared per ASTM D6386, usually by cleaning then brush-off blasting per SSPC-SP 16.

Can alkyd paint be used on galvanized steel?

Not directly on the zinc. Alkyd saponifies and peels there. If an alkyd topcoat is required, it goes over a primer compatible with zinc, per the data sheet.

Does brush-off blasting remove the galvanizing?

It should remove only a very small thickness of zinc. A fine abrasive, reduced pressure and checking the remaining zinc thickness avoid exposing the steel.

How is an area where the zinc is gone repaired?

The area is prepared, then repaired per ASTM A780, often with a zinc-rich coating, before the primer and topcoat of the duplex system are reapplied.