A colleague of mine, Stephen Heigh, has challenged himself to completely fill up a 100 page sketchbook by the beginning of the school year. Stephen started mid July and has created sketches on location as well as from reference. He starts with tonal pencil and then dry brushes with acrylic to create a sepia underpainting quality. Here are some of his excerpts.
This process of multiple studies sharpens your eye and speeds your execution. It can also become extremely therapeutical. I host two online painting groups and am now starting to believe that a "sketch off" might be introduced sometime in the next few months.
Until then....good luck finishing the sketchbook Stephen! Nine days left. No pressure.