The hammerite will probably adhere well to the rust, the thing is then how well does the rust adhere to the metal base materiel. You might end up with the Hammerite flaking off.
When painting directly on rust one have to be very sure that the whole surface is sealed by the paint so that no water or air can pass through otherwise you might make very good conditions for crevise corrosion underneath the paint.
I have also heard that its fairly brittle, so any impact would chip it.
I would clean it with needle gun as a minimum befor painting.