Though many specifiers are familiar with one type of corrosion protection or another, few realize the intrinsic value of utilizing two corrosion protection systems together, known as a duplex system. A duplex system is formed by painting or powder coating over hot-dip galvanized steel. When used together, the corrosion protection of the two systems combined is far superior to either protection system used independently.

Painting or powder coating hot-dip galvanized steel requires careful preparation and a good understanding of both systems. Many products have utilized a duplex system successfully for decades; automobiles and radio towers are two examples. When the galvanized surface is prepared correctly, paint and powder coating adhesion is excellent, and the duplex system becomes a highly successful method of corrosion protection. However, it is important to execute proper preparation to ensure this success.

Past experience provides excellent historical data for how best to achieve good paint or powder coating adhesion. Studying past adhesion failures and successes led galvanizers, paint companies, researchers, paint contractors, and other sources to create the specification ASTM D6386, Preparation of Zinc (Hot- Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, and begin drafting a specification on preparing galvanized surfaces for powder coating.

Revised in 2012, this 12-page publication provides an overview of duplex coated applications, while detailing procedures for successful adhesion. Topics addressed include why hot-dip galvanized steel is painted/powder coated, benefits of duplex systems, pre-treating, surface preparation procedures, and recommendations for painting/powder coating new or weathered surfaces.