If you went to school for autobody you should know that you just can't sand rust away and bondo over it. You have to either sandblast it or chemically treat it, or replace the whole area with new metal. If you sand it and just use body filler(bondo) to go over it, there is still rust there and it will come back. You have to fix it right, otherwise don't paint over it. Infidel.... do you see the coolant puddling anywhere like near your injectors or anyplace on the intake manifold?