Starting pastel paintings with a thin, wet, loose underpainting is something many artists enjoy. Depending on the medium used and the surface it applied to, it can have a variety of appearances.
Personally I have utilized some form of underpainting from the earliest years of my painting adventure. Thin watercolor and oil washes have becoming two of my favorites. Two things need to be analyzed when choosing your means of underpainting: how the medium will respond to the surface: and how it will interact with the pastel. Do a little research and experimentation on your own before committing major efforts to a procedure that may prove to be non-archival.
What lead me to experiment with very thin washes of oil paint was the introduction of acrylic-based sizing and binders in the manufacture of pastel surfaces. These allow for no migration of upper layers to the substrate surface below: in essence, isolating it from any harmful chemical interaction. Papers such as Wallis sanded paper even state that they accept oil paint. I don’t advocate thick applications of oil. Besides taking a major amount of time to dry, it would introduce a considerable amount of oil (commonly linseed oil) that could negatively interact with the pastel. My working procedure is to thin the oil colors to the consistency of weak tea using a highly refined mineral spirit like Gamsol by Gamblin, or Turpenoid by Weber. I apply these very thin washes with a brush, allowing them to run and interact to produce an interesting underpainting (see the example above). This is merely a stain and I can’t stress enough how thin it must be!
After the mineral spirits evaporate, which happens very quickly, pastel can then be applied. You may ask: why oil? Why not just use pastel spread with mineral spirits? The reason is ease of application. I can better control the placement of color and bleeding of the colors with tiny amounts of oil paint mixed and made wet on a separate palette. Pastel made wet on the painting surface is much more unruly. It’s nearly impossible to tell the two apart, much like a watercolor underpainting compared to wet pastel. Since many pastel artists work in other wet media, they are often more comfortable getting a painterly underpainting by applying the initial color with a brush, but it really is just a matter of personal choice.