Here is an example of what skim coat is. I use a drywall blade to apply joint compound  over an wall that has already been painted.  The idea is to smooth the walls and sand and prime then paint. This creates a new looking wall with no bumps, defects, unevenness and un-sightly walls. Call me @ Six 1 Five-Five 86-5400 or email me @ judthepainter at (@)