Painting Galvanized Steel

How to Pretreat Galvanized Steel for Painting

Q. I am the engineer at a metal fabricating plant. We have a conveyorized line to pretreat the metal, mostly cold rolled steel, before painting in a three-stage iron phosphate immersion system. We have the flexibility to apply most of the coatings available today, including powders.

One of our customers ordered enclosures made of galvanized steel for a special job. I am not sure how to pretreat this material, although I have had many suggestions. My painter claims he always washes galvanized steel with vinegar before painting. Is this the best thing to do or is there a better way? 

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A. There are more home remedies for pretreating galvanized steel than there are cures for the common cold. Most of them, such as washing with vinegar, muriatic acid, copper sulfate or apple cider are very unreliable. Painting galvanized steel presents a problem because of its treatment at the mill and the reactivity of the zinc on its surface. Waxes and oils are applied at the mill to prevent “white corrosion.” Zinc’s reactivity with certain coatings, such as oleoresinous and oil-modified alkyd-based paints, can cause adhesion problems. Before painting, the galvanized surfaces must be cleaned and passivated. This is best done by solvent wiping, alkaline cleaning or detergent cleaning followed by a zinc phosphate pretreatment. Certain iron phosphates formulated to work well. Your pretreatment chemical supplier can tell you if your present iron phosphate solution will work with galvanized steel. After wax and oil removal, treating with a solution of phosphoric acid is another passivation method. Excellent results can be obtained, after cleaning, using a phosphoric activated vinyl wash primer such as MIL-C-15328, direct to metal with no other pretreatment. 

How to Coat Galvanized Steel for Long Term Performance

Q. We have been painting solar collector assemblies, not the collectors themselves, but the structures that support them. These structures are fabricated from galvanized and plain steel parts, depending on their location in the assembly. Under certain conditions, the paint must withstand temperatures up to 300°F for extended periods of time. We have been applying a low-gloss black enamel to these structures.

In this application the paint must last for several years in service without peeling, flaking or other deterioration. We have been getting customer complaints about paint peeling and flaking off the assemblies. What is the best way to solve our problem? 

A. Your problem has a two-part solution. The first part is the choice of a suitable surface pretreatment and the second part is choice of a suitable paint. The best pre-paint treatment for both galvanized steel and plain steel in outdoor service is a zinc phosphate.

Since you did not describe the type of paint you are applying, I can't judge its suitability for this application. The best coating to use is a high- temperature, corrosion- and weather-resistant paint. These coatings are commercially available. Your present coating may have the required performance characteristics. Discuss this with your paint supplier and ask for a performance data sheet. Then ask if the paint can be applied direct-to-metal. If it cannot, it will require use of a primer.