The rear quarters i usually spray with a heavy under coat like the factory had . The trunk floor i would use the tar type mat ,the tops of the gas tank had this from the factory and when the trunk mat is in place you can't see it .
lizard skin sound deadener works just about everywhere you can spray it. dynamat is also good for those areas you dont want to spray. either one will protect the top of the tank more than just the factory mat and original deadener will.