Reel to reel tapes and even cassette tapes are a mylar plastic with a metalic oxide coating on one side. This gives it the dull appearance. Often referred to as magnetic tape, it is this dull side that has the signal on it. The dull side is the side that contacts the head as the tape goes past to the other reel when recording or playing back. No reel to reel tapes were made with magnetic coating on both sides. Some better tapes were "back coated" to give the pinch roller and capstan better grip for tape handling, but that side was never meant to be in contact with the tape heads.
Hope this helps.
Hope this helps.