I have started to put in a light glaze of color to the underpainting. I am using a simple method of application called a "couch". I apply a few dabs of thick stand oil in the area I wish to paint and then rub paint thinner into the oil. (I use Gamsol mineral spirits.) This provides a nice slippery surface to work into and dries within a days time. The stand oil helps keep the paint conditioned and slightly fatty so that it won't crack over time. This is only the first layer of paint that I am putting on right now. This gives me an idea how close my colors are at so that I can tweak them later.