I am trying to remove/cover the textured ceiling of a 60s home we are beginning to renovate. This particular texture is plaster that was "newspapered" into raised ridges. I am planning on covering it with knock-down and wonder how much of the old texture I need to remove. It is like Concrete! I am using and orbital sander but it is slow and muscle-killing work. Do I need to to sand it almost flat or can I just knock the high spots off and cover with the knock-down. Thanks!