Paint failure mechanisms are somewhat complex when paint is applied over hot-dip galvanized steel, otherwise known as a duplex system, and they may be manifested as peeling, flaking, or blisters. And regardless of the age of the galvanized steel (new, partially weathered, fully weathered), the cause is either inadequate surface preparation or using a paint/powder coat system not compatible with zinc. First, lets examine inadequate surface preparation. Although the zinc coating of newly galvanized steel may look clean and ready for paint or powder application, the surface is actually quite smooth and requires cleaning and profiling to create a good mechanical bond. If the galvanized steel is partially weathered, i.e. 2 days to 12 months, depending on atmospheric conditions such as moisture, it has developed zinc corrosion products of zinc oxide and zinc hydroxide and likely also contaminated with organic dirt and grease. The surface must be cleaned of contaminants and then profiled by sweep blasting, wash primer, or acrylic pretreatment. If the galvanized coating is fully weathered it is usually matte gray in color due to the formation of a zinc carbonate film on the surface. This film is tightly bound to the galvanized surface, has a fairly rough profile, and generally requires only cleaning to be suitable for paint or powder application. 

If the galvanized coating is not clean and have a good profile, peeling and flaking of the paint or powder may occur. Regardless of the age of the galvanized coating and the cleaning and profiling steps taken, the surface must be completely dry before application of paint or powder should be considered. Particularly for powder coatings, if the galvanized surface is not dry and powder is applied on top, the moisture will react with the zinc to form zinc oxide. During the curing process of the powder coating, moisture trapped in the zinc oxide may expand and be released (called outgassing), causing pimples or blisters in the powder coating. Secondly, some paints are not formulated to be applied over a zinc coating and they react with the zinc to form corrosion products and/or films which act as inhibitors of adhesion. Most reputable paint manufacturers have paints uniquely designed for application over galvanized coatings. 

See Also:
Guide to Preparing Hot-Dip Galvanized Steel for Paint
Duplex Systems