Flash rust is a significant problem for all water borne coatings applied over steel. What is flash rusting? Flash rusting is corrosion from steel bleeding through a coating creating a stain within hours of the coating application. Long term corrosion is not flash rusting. Flash rusting occurs when a thin film of water resides on the surface of steel for a brief period of time causing corrosion to take place. The corrosion products are often soluble and can migrate within a coating to produce a stain. It should be noted that steel can be found in many coatings applications other than the main substrate.