Painting Galvanized Steel: Match the Prep to the Age of the Zinc

Wally Bates

Sep 26

2 min read

Galvanizing is itself a coating, so painting galvanized steel means coating a coating. Handrails, canopies, stair stringers and exposed structural steel on architectural projects are often galvanized, and paint peeling from them is a common callback.

Underside of exposed architectural steel framing

Why galvanized steel is difficult

It is smooth and dense. There is no profile for the coating to grip.
Zinc is reactive. Within days it begins forming zinc oxide and hydroxide, the white, powdery bloom known as white rust. That layer is loose and alkaline, and paint will not adhere to it.
It is often passivated. Many galvanizers apply a chromate quench or passivator to slow the bloom. Paint will not adhere well to that either.

Prep depends on the age of the zinc

ASTM D6386 sets out preparation by condition:

Newly galvanized, under about 48 hours: solvent or alkaline cleaning, then coat.
Partially weathered, about two days to two years: the most difficult stage, with oxides present across the surface. It calls for a sweep blast, an acid wash or an etching primer.
Fully weathered, past about two years: mostly stable zinc carbonate. Pressure wash and run a test patch.

If blasting is needed

The standard is SSPC-SP 16, brush-off blast for galvanized and non-ferrous metals: low pressure and fine abrasive to produce a 1 to 2 mil profile without stripping the zinc.

A practical rule

Tnemec's guidance is to assume every galvanized surface has been passivated, and to tape-test a mock-up after a week before coating the full job.

What to check in your spec

Ask the fabricator whether the steel was quenched or passivated, and when it was galvanized.
Reference ASTM D6386 for prep by condition.
Specify SSPC-SP 16 where a sweep blast is required.
Require an adhesion mock-up before full application.

Further reading: Tnemec Technical Bulletin 10-78 R2, Surface Preparation and Coating of Hot Dipped Galvanized (Non-Immersion). Standards: ASTM D6386, SSPC-SP 16, ASTM D7803.

Talk to a consultant. Atlantic Coating Consultants helps owners, engineers and contractors in PA, NJ, DE, MD, DC, WV and OH with specification writing, on-site condition assessment, training, product recommendations and field support. Call 484-644-5303 or email atlantic@tnemec.com.