See this thread for same topic:
http://www.vansairforce.com/communit...ing+powdercoat
I think there are two options and both require removing the old powdercoat and rust first.
Option 1 is to take to a powdercoater and get the parts re-coated. (Maybe difficult for you because the supports look like they are riveted on.)
Option 2 is to paint with an epoxy primer and then a good quality topcoat.
In order to remove the old coating you can use paint stripper. Then sand off the rust back to clean bare metal.
Good luck.