You might respond that painters should be painting, not drawing. But drawing is all about getting shape relationships right -- exactly the same thing that painters should worry about. As a painter, though, I tend to create these relationships with a brush rather than a pencil, and with shape alone rather than with line. In fact, I rarely paint lines; they come into existence automatically when two shapes share an edge.
It's hard for a student to learn to draw when he's also learning to paint. Getting perspective right is tough when you're also trying to deal with color and the sometimes-intractable nature of your materials and equipment. Best is to make the drawing practice a separate event, and with pencil and paper.
Lately, I've taken to sketching after I've done my demonstration painting and I'm going from student to student, offering help at the easel. If the class is small and I can spare the time, I'll take my pad around and do quick, 5-minutes sketches for my own amusement and practice. (The pad is also handy to have so I can draw on it to illustrate a point for the student.)
Below are some of the sketches I did recently at a workshop on Ministers Island, in St Andrews, New Brunswick. They are rough, but I'd rather work out the lines of some of these very complex barns on paper before I take paint to them! (I've also included a photo of one of the barns, build by Canadian rail magnate Sir William Van Horne.)
