So the walls make a 135 degree angle? Or do you mean a 45 degree outside angle. The inside (135 o) angle can be done with paper tape, but it's a little hard to keep the line straight. There is a product called No-Cote for finishing drywall corners that will do an excellent job on either inside or outside angles and is much easier to apply and tougher than tape or corner bead. It may be a bit tough to find though. They advertise in Journal of Light Construction all the time. If you are interested and can't find it e-mail me, and I'll look up there number.