Investigation of Staining of Newly Coated Steel Framing

Reddish-brown staining appeared on pre-primed steel framing members soon after the application of the topcoat. Despite the primer manufacturer’s early hypothesis that surfactant leaching of the topcoat may have been the cause, field and laboratory investigations indicated that water-soluble corrosion products were being mobilized and deposited on the surface due to the presence of moisture. Pre-primed steel delivered to jobsites can begin to rust before receiving the topcoat. Areas of latent corrosion where the primer thickness was low, along with the framing that had been stored outdoors, likely led to corrosion staining after topcoat application.