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Hot dip galvanized coatings are sometimes required to be painted for decorative reasons, to provide an identifying colour, or to enhance service life. In contrast with organic paints, which are degraded by solar radiation (UV), hot dip galvanized coatings are unaffected by sunlight, so over-painting is not usually required to extend service life in this circumstance.
The AS/NZS2312.2:2014 Standard includes a detailed section on the design of duplex coatings (paint over HDG), with two performance options for durability (aesthetic and corrosion).
Further, the total life of a properly specified, applied and maintained duplex coating system is significantly greater than the sum of the lives of the HDG coating and the paint coating alone (by 1.5 – 2.3 times, depending on the environment).