Reasons for Duplex Coating

There are seven good reasons to paint or powder coat hot-dip galvanized steel:

Extended Durability – in what is called the synergistic effect between galvanized steel and paint, the zinc of the galvanized coating is a barrier protection to the substrate steel, cathodically protects it, and provides underfilm corrosion protection of the paint. Synergistically, the paint provides a barrier protection to the zinc coating.
Economic – because the zinc extends the life of the paint, less maintenance is required over the entire service life.
Ease of Repainting – when the paint of a duplex system completely weathers, the exposed zinc coating requires minimal surface preparation when repainting.
Aesthetics – desiring a color to match surrounding structure or for branding.
Safety Marking – the extremely high cost to maintain a very tall structure often means galvanizing is a requirement but those same tall structures are mandated to be painted safety orange and white for high visibility to air traffic.
Extending the Life of Previously Galvanized Steel – large in-place structures often cannot be regalvanized after decades of exposure and so a logical means of keeping the structure in service is to apply a paint, often an inorganic zinc-rich paint which provides barrier protection and some cathodic protection.
Color Coding – steam, water, and other fluid piping systems are often painting in industrial facilities to distinguish them for quick repair and safety concerns