ASTM A588 Grade A / A606-01 Type-4 with a minimum Atmospheric Corrosion
Index of 6.0

+ 28 refers to the number of days the skin has been exposed to the elements resulting in the initial weathering process (the initial oxidation is not the final “scale” that develops which is a much deeper red/brown/purple hue and is actually the protective scale.  At +0 days – the metal is bare and unoxidized.  The wonderful thing about weathering steel is that it develops a patina that is totally dependent upon the environmental conditions present… which means the building starts to reveal the prevailing patterns of weather, ie. which way the wind blows the rain most generally… etc…