As a general rule you will need between 1.5ml and 2.5ml of solution to coat a surace area of 8X10". The best coating brush is the Richeson, otherwise known as the "Magic Brush" Good Hake brushes work well, but absorb a lot of the sensitizer to you will need more solution with these kind of brushes.

The coating technique described by Aggie is very sound and will give good results regardless of the brush your use.

Don't ever leave puddles of sensitizer on the paper. Brush until all of the sensitizer is evenly distrubted and there are no more puddles, then leave for about 3-5 minute flat (better with a fan blowing on the sensitized paper), then hang to dry.