Why are the edges of my painted galvanized roof rusting?

The edges of the metal are areas of unprotected carbon steel. Factory cut edges will last significantly longer than field cut edges because more carbon steel is exposed during field cutting operations.

The edges of the metal are areas of unprotected carbon steel. Steel is protected from corrosion by application of zinc to the surface of the carbon steel, commonly known as galvanizing. The zinc is sacrificial therefore helping to protect the steel below. When factory applied painted finishes are also applied, additional protection is provided by creating a barrier to corrosion. At cut edges, zinc will protect the steel for a time but eventually begins to deplete itself in the area of the cut edge. Once depleted, edge corrosion can begin to creep from the ends resulting in red rusting and the paint will oftentimes peel back in areas where the zinc is no longer present. Factory cut edges will last significantly longer than field cut edges because more carbon steel is exposed during field cutting operations.