Shading, shading, shading: In the underpainting, establishing a likeness, laying down color and shading are the most important goals to achieve. The latter is the most important information for this collage-application road map. The shading of the painting will help to guide the paper gluing, so you are creating volume with your paper brushstrokes.
Where is the darkest dark? Where is the lightest highlight? Where is the medium value? How does it gradually transition to light and to dark? How light is the light side? What is the direction of the light source? These are all questions that are sorted out and answered in the underpainting stage so that the collage application can simply follow these established shading guidelines.
Once you have established an underpainting with a full range of values (from the darkest dark to the lightest light and every value in between), you will be ready for collage application.
Underpainting establishes color, shading and composition. A full range of values is important but details can be loose. We get more detailed in the collage application, but we use the values established in the underpainting as an accurate guide for shading (which creates volume) in choosing papers to glue on top.
