It looks like plastic film, but nothing like any tank coatings I have seen. The coatings never last and rust usually comes back. If you are interested in replacing the gas tank with a new one, Dansk makes a very nice 73.5 tank with the internal baffles and swirl pot to improve fuel delivery, along with ability to hold full size spare. About $550.
http://forums.pelicanparts.com/porsche-911-technical-forum/1128886-73-5-fuel-tank.html