My trouble started when I
decided to remove a wall-to-wall mirror in order
to tape under it to patch an adjacent newly installed section of drywall. The mirror was glued to the wall with mastic, and removing it stripped off the top layer of paper from the drywall behind. I didn't
save the mirror, though it came off in one piece - pulling the mastic from the back removed the silver
plating. The drywall behind
was white, *perfectly* smooth, must have come from the factory primed. I mudded in the shorn paper, and the top (white) surface
wrinkled badly around the mudded area, so I tore off
the wrinkled part, way back.
Maybe I'll just tear off the entire surface, and use
the next layer of paper for mud. However, the next layer below also wrinkled
when mudded. So I bored a 1/2 test hole, and filled it with mud. A few hours later, the surrounding paper and
gypsum (I assume) had warped and crumbled. The
stuff can't be mudded, either on the surface or patching gouges. I've patched hundreds of drywall
dings, and never seen anything like this. What is this stuff, and what I do about it?