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Weathering steel is not typically painted, because it produces a thin rust layer that protects the rest of the steel from further corrosion. Would painting weathering steel be an acceptable additional line of protection, or would this cause some unforeseen complication?

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According to Maintenance Coatings of Weathering Steel , published in 1995 by the US Department of Transportation

From page 11 of the report:

Painting of new uncontaminated weathering steel is generally not considered a problem. Test fence and laboratory data developed by the paint industry have indicated that conventional coating systems such as oil alkyds and epoxies will perform comparably on weathering steel and on carbon steel if the degree of surface preparation is equivalent.

The major problem faced by highway departments and other owners of weathering steel structures is protecting weathering steel that has corroded in the presence of chlorides and other contaminants. Conventional cleaning techniques such as dry abrasive blasting do not remove the chlorides, which apparently penetrate the bases of pits in the steel. The performance of standard highway coatings such as oil alkyd, epoxies, and zinc-rich systems over chloride contaminated steel has not been satisfactory.

Additionally, the Florida Dept of Transport is recommending using:

weathering steel for new steel bridges in suitable environments. Use a single coat of inorganic zinc paint system for extremely aggressive environments. Where higher aesthetics are required, use a 3 coat inorganic zinc paint system with clear coat finish. The use of the 3 coat system with clear coat finish should be an exception

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