POR15 can be used on rust. But the best base for POR15 is the bare metal. If any paint or grease or oil is left on the metal, POR15 will not permanently joining with the metal.
I have painted my frame with black frame-paint because this paint is more insensitive and longer brushable. Painting the whole frame takes hours and POR15 needs a second layer within 4-12 hours. After that time the second layer will not connect adequate to the first layer.
I think there is no need to use POR15 on the frame and it makes more work than necessary.
