This is where you run into trouble, blindly trusting what AI comes up with. Try googling ferrous oxide- it's a black, powdered form of iron oxide. "...one of several iron oxides, it is a black-colored powder that is sometimes confused with rust..."

You really have to dig a little deeper and double check everything you find on-line these days.

And again, where would you come up with enough rust to make enough paint to cover a barn?