Help! I found that tin foil damp proofing stuff on my walls too!

I stripped off the old wallpaper in the living room and found that the area around the window had the tin foil stuff about half way up the walls. Bits of it came off with the steamer, but most of it didn't. I had been planning to emulsion the walls, but what can I do now? Is there a way of getting the foil off, or can I paint over it or is the only solution to paper over it? My house was built in about 1880 and the plaster is not too bad for its age. Any help or advice would be appreciated. Many thanks.