Okay, so let's listen in on a conversation somewhere in our nation's heartland:

"Say, Jim, that's a mighty fine new barn ya' got there. Ain't ya' gonna paint it?"
"Well, I would, Bob, but I ain't saved up enough rust yet! Been leavin' my equipment set outside, but it's a mighty slow process..."

Now can you see the absurdity? Yes, we all learned in school that rust is iron oxide, but some of us stuck around to learn that 'iron oxide' is a generic term for a whole host of different compounds. And yes, some artists, modelers, and other craftspeople do make their own paint using rust. But I don't think even the US Navy could generate enough 'good, clean rust' to start a side gig making barn paint...

Furthermore, I don't know anyone foolish enough to paint a car or any machinery with barn paint, though I believe International Harvester did try it for several years...