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Chinatown sculpture
Example of a completed duplex system

Similar to painting over black steel, surface preparation is critical when painting or powder coating over the zinc coating of hot-dip galvanized steel. However, before any surface preparation takes place, the galvanizer should be notified the steel will be painted. With this understanding, the galvanizer will forego any post-treatments that could affect paint adhesion and take special care in delivering a surface suitable for painting or powder coating.

Assessing the condition of the galvanized steel surface is the next step in preparation. Newly galvanized, partially weathered (exposed to the environment for anywhere from 2 days to 12 months), and fully weathered each require different surface preparation (cleaning and profiling). Newly galvanized steel has little or no organic contaminants and can be prepared by carefully grinding any bumps, runs, and drips, rinsing, drying, and profiling of the relatively smooth surface. Partially weathered galvanized steel may require some grinding and is then typically cleaned of organic contaminants by an alkaline solution, acidic solution, or solvent cleaning, rinsed, dried, and profiled. Fully weathered galvanized steel requires only cleaning and rinsing because the zinc carbonate patina formed over time is tightly bound to the weathered surface and relatively rough, creating a good profile for paint adhesion. Profiling of newly galvanized and partially weathered galvanized steel surfaces may be done by sweep blasting, applying a wash primer, or applying an acrylic pretreatment.

A complete description of the required surface preparation can be found in ASTM D6386 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting and ASTM D7803 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surface for Powder Coating. When is the best time to paint or powder coat over hot-dip galvanized steel? The short answer is when you need to paint or powder coat over the hot-dip galvanized steel. It can be done successfully at any time. There are just different surface preparation requirements and costs associated.

See Also:

Guide to Preparing Hot-Dip Galvanized Steel for Paint

Duplex Systems

Surface Preparation Methods for Adding a Duplex System to Galvanized Steel

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