The practice of under painting is almost as large an area of study as oil painting. The purpose of an under painting is to establish the composition and overall light/dark pattern while initiating the development of volumes and substance to the forms. An underpainting allows you to envision the totality of the pictorial idea. In addition to correcting drawing errors the under p ainting's primary purpose is to fix the all important Notan, which is the Japanese term for light/dark harmony. Generally speaking, there are three types of underpainting: there is the fully worked up monochromatic grisaille. This is the academic approach where little is left to chance. However, the classical grisaille of mixed white and black possesses an Achilles Heel. As the white paint ages it becomes more translucent and the black paint in the grey mix will dominate. Hence the overall picture will darken significantly, especially if there was extensive correcting of t...