One method I use for doing larger surfaces is to thin down the mud like you would for stomping a ceiling. I then use a roller to "roll" the mud onto the surface to be skimed or "re-surfaced".
Then take your 6" or 10" blade and "take it off". Be sure and have your blade at an angle...you want to leave a small paper thin amount behind...(unless you like sanding

) Work some of the mud with the knife upto and down to the area's you cannot get with the roller. IE corners, ceiling / crown molding or baseboards.
The thinned mud will not set-up as fast and allows you to work a larger area at a time. (effeiciency=profits) Plus the thinned mud provides for less wrist fatigue. I use the Sheetrock USG light blue lid "Topping"
You should be able to re-surface a wall with a minimum of 3 coats upto about 6 for the worst conditions like a nasty wallpaper removeal where lot's of the drywall paper has come off in the process.
I've also done this on repaints where a homeowner has painted prior and has left a heavy stiple, ropes etc.
Put it on...take it off....end results are smooth wall with minimum sanding.
-Martin