Walls and Ceilings - Waves under the Mud!!

I recently removed several layers of wallpaper from an old sheetrock wall. In removing the last layer I also removed some of the top layer of "paper" on the sheetrock since the previous owner did not "size" the walls correctly. I cut away the loose paper, however, when I tried to apply a skim coat to the area where the top layer of paper was missing, the paper remaining appears to be shrinking as it is causing "waves" under the mud!!

My question is do I need to apply some type of material over the area that needs to skim coated first? remove the rest of the paper and expose the plaster filling, then skim coat it? or is there some type of process that I am missing!!

Some of the areas are somewhat large - 18" x 18".

Any suggestions would be appreciated!!

J.

Doesn't sound like you aren't to the drywall yet. Sounds like its still some of the wallpaper backing. Or, worse, it could be that you have masonite, not drywall. I had that in my first house. The previous owners had put masonite on one wall. This masonite looked like wallpaper. I was trying hard to remove what I thought was wallpaper. What a mess!!