Smooth a badly damaged wall
Drywall. Thin (1/4 inch is available) drywall. It can be screwed to the subsurface or glued if there is not easy access to wood or steel framing. Tape the seams and the edges, a few coates of joint compound, and it is smooth sailing. 

If the subsurface is not level enough, a few shims, also glued to the wall should let you get a fairly flat surface.

This is not structural, just a solid surface for paint or wallpaper.