As Dave S has said, clean and prep as you would the body. POR-15 applied over clean (no rust) really doesn't work worth a darn. Also, it is photosensitive - will deteriorate and change color without a top coat. If the car is worth it, then an epoxy primer with a good multi coat single stage urethane paint will do the best job of preservation - anything else in Florida is a waste of your time and money. Or put down a fresh coat of premium undercoat and forget about it. Also, the interior floor should also have a good epoxy prime as well.