Yup, make a paste with the BKF, glob in on, wrap in a damp paper towel, wrap plastic around it and tape it. Come back tomorrow, clean it with water with a little baking soda to deactivate the acid. Get some rust killer, little can in the paint dept at the hardwar store, and a small brush, just coat the rusty bits to finish killing. Add a little clearcoat spot on top....carefully.