Today I worked on the grisaille stage of the underpainting, which is a value-only underpainting. The previous stage uses raw umber and ultramarine to make a neutral color grey, and uses the white of the panel for the lights. This is called an open grisaille, because the white of the panel shows through.
This second stage is a closed grisaille. I use the same raw umber and ultramarine blue for the neutral dark gray, but I also use flake white (cremnitz white) for the lights. When it is done the white of the canvas will not show at all, the surface will be covered by paint.
I used to only paint an open grisaille layer, and then go right to color. But I have found it saves me a lot of time (and anguish) in the later stages of color if I take the time to make a complete value painting in closed grisaille first. The open grisaille is just too transparent and textured to behave well as an underpainting for my needs.
I also spent some time wet sanding this layer with my underpainting medium before I began painting. This removes dust that may have embedded in the previous layer as it dried, and makes it easier to paint on the dry surface. Putting down a layer of medium to paint into is called a "couch".
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