When working with watercolor and pastel, it's important to keep in mind that watercolor is transparent with a painterly quality, while soft pastel is graphically opaque. With proper planning, these contrasting qualities can complement each other so that the best is achieved from both mediums. Like watercolor and gouache, acrylics are also water-soluble, making them compatible for use with pastels. Most acrylics will overpower pastels if the two are combined directly, so acrylic mediums or gels must first be used to dilute the acrylic pigment. Once diluted, acrylics lend pastels the body necessary for impasto textures.
Acrylics also help make the surface of a pastel painting strong and durable.
As with watercolor, you can use a finished acrylic underpainting as a base for further pastel highlights. When drawn directly on acrylic, pastels can exploit the present surface textures. And since acrylic is such a strong support, it can withstand the surface abrasion of pastels as well as the use of fixatives. Pastel paper alone is not a strong enough support when these two media are used together. Instead, use a 2-ply or thicker illustration board so that the surface does not buckle when wet.