Starting pastel paintings with a thin, wet, loose underpainting is something many artists enjoy. It sets up a foundation on which to respond with the subsequent application of pastel. Though not meant to be the finished painting, the underpainting often plays a major part in the final appearance. 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. Over those years, I have experimented with a variety of surfaces and media. 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.