I think most of us got our start painting because of Bob's infectious Joy of Painting. It was contagious. If you were like me, I strove initially to do the painting in one sitting in less than 30 minutes. We all eventually figured out that we just were not that skilled as Bob. Once I figured that out and slowed down I improved dramatically.
So what is underpainting and how does it work using the wet on wet technique?
Simply put - Underpainting is a base layer of a "draft" of the painting sketched out directly onto the canvass. I like to use a graphite pencil to lay out the basic shape of a mountain or cabin. In traditional painting it is easier since there is not a base coat of liquid white and you can just draw a sketch on the canvass directly. That is ok no reason to panic, I generally put the very thin coat of liquid white on first and then I'll take either a lead or graphite pencil. I'll trace out the basic mountain shape, or cabin etc right on to the canvass directly over the liquid white. I find it especially useful with cabins to get the angles correct. Should you get the shape, size or angles wrong- you just take the big brush and wipe the tracing away and start over ( happy accident ;-)). I also found that when it comes to layering colors, shading, foreshadowing and highlighting I like using base underpainting to see if the colors work
Hope that helps!