Just use some outdoor Rust-Aid. It's in a red bottle, and I'm pretty sure it's available at your local Walmart. It takes it right off, with no effort. Dab some on a rag, and scrub with moderate pressure for a few seconds and they disappear.
To prevent mine from doing it again, I sanded them down with a power-sander, taped off the surrounding areas, then primed and painted. I used Rustoleum paint and primer, and the streaks have yet to return.