I would use a rust paint first to stop and seal the rust then put your rubberized coating. Up here in the north if you used a thick rock guard or undercoat the thick coating would hold in the moisture and the metal will rust underneath your nice thick coating and could go unnoticed until it blisters and by that time you could probably poke a hole. I have started using a paint called chassis saver and so far seems to work fairly well at sealing metal that has surface rust .
