I begin the painting by mixing a half paste of flake white and a gel medium.  It is translucent so I retain all the form of the drawing and I can build the portrait’s dimension with light and shadow.  I paint this half paste in a thin layer for the foundation.  I keep it smooth at this stage by knocking it down with a fan brush.  This helps to create a surface that will easily accept the next step which I usually try to do immediately.