Flash rusting is signaled by the presence of small rust spots on the paint film, or sometimes by a tarnishing of larger areas, and usually appears shortly after application and before the coating is dry to touch. It generally results from slow drying conditions, especially at high humidity, and is caused by the migration of corrosion products through the paint film.
It generally does not affect long term performance, but detracts from the coating’s appearance. Flash rusting can be eliminated through proper formulation of the waterborne acrylic coating. Inorganic salts such as sodium nitrite are typically added as flash rust inhibitors. Avoiding application under extremely high humidities is also recommended.