Duplex system corrosion protection, i.e. painting or powder coating over hot-dip galvanized steel, is utilized for maximum corrosion protection in very aggressive environments, color coding, aesthetics, and/or extending the life of galvanized steel near its time to first maintenance. This combination of coatings delivers superior cathodic and barrier protection from the galvanizing and some barrier protection from the paint or powder. Once the paint or powder is damaged or wears thin, the underlying zinc provides decades of protection.
Surface preparation of a galvanized steel surface to be painted or powder coated is somewhat unique, as the age and surface condition of the zinc coating determine whether chemical cleaning or sweep blasting is required. Just as is the case with any coating, proper surface cleaning can deliver high quality duplex coatings, and in fact, has been done for over 30 years. The proper surface preparation techniques are found in the ASTM specification D 6386. A summary of the preparation can be found in the Paint Guide.
