Comment by jasonwatkinspdx

Not quite right.

Iron oxide is anti-microbial. So a barn painted with it as a pigment will last longer vs mold and such.

It's also why large ships all have that red paint color below the waterline. In that case it's copper oxide, which helps slow barnacle growth and similar.

Back in the age of sail they even went as far as copper sheet cladding to make the wooden hulls last longer. Copper oxide pigment emerged toward the end of the age of sail / beginning of the steamship era as a more practical alternative.