Wet storage stain is often light and superficial, despite the presence of a bulky corrosion product. In the vast majority of cases, wet storage stain does not indicate serious degradation of the zinc coating, nor does it necessarily imply any likely reduction in the expected service life of the product.

Medium and heavy wet storage stain can cause damage to the galvanized coating and can eventually destroy it if the wet storage stain is left untreated. In advanced stages of wet storage stain, the typical white or gray corrosion products may become black in color. When this occurs, a significant amount of coating has been lost to corrosion and the piece must be stripped and regalvanized.

See also:
Wet Storage Stain Publication
Wet Storage Stain vs. White Rust Dr. Galv Article