EXT wrote:

Sanding with a large sandpaper holder, either with or without a vacuum
serves to flatten the mud, making a smooth surface across the sheets of
drywall. Unless you have kept the mud buildup to a minimum with no hills or
bumps a wet sponge will only smooth out the bumps into gentle hills and not
take off any excessive mud, which is needed to get a dead flat appearing
joint. The gentle rolling hills will be visible after the first coat of
paint.


Absolutely right - so if you sponge, be sure to get something BIG like
the ones Lowes & DoItBest hdwe sells:

http://doitbest.com/Sandpaper+and+em...sku-303895.dib

and use it in a circular motion like you're washing a wall.