skim coat plastering is applied over blue board approximately 3/32 inch thick. the whole surface is covered compared with drywall where just the seams, nailheads and corners are mudded. drywall needs sanding, 3 coats of compound. plaster is one coat and troweled to a glass surface, no sanding. plastering is an art relative to drywall. I am the plaster geek because I am a plaster specialist. I can blend any patch so it cannot be seen to be patched. any surface can be skimcoated, painted surfaces, stone, old walls can be skimmed over and made smooth with plaster.