After Rubens portrait underpainting

The old Masters often used an underpainting to lock in shapes and values.  One method is called a wipeout where you draw contours and and shadow shapes onto the canvas, then apply a thin layer of raw umber or burnt umber.  Using cloth from an old t-shirt you wipe out the lightest values first and then the other light and mid-tones.  You can add back in darks that got wiped out using a brush.  Below is a quick study I did after a portrait by Rubens.