Whether you strip off the old wallpaper and paint or not, make a flat valley over the crack about 7.5 cm (3 in.) wide and 3 mm (1/8 in.) deep using a paint scraper. This will create the space necessary for strong plaster and fiberglass tape. Moisten the existing plaster and embed the tape in a layer of fresh plaster. Make sure that the tape is slightly recessed from the wall surface because it will be difficult to sand. Finish the job with a couple of coats of drywall joint compound for a smooth finish flush with the wall.