Originally Posted by gdlantz
Using just soft pastel without water or alcohol also considered underpainting? Thank you Ed and Deborah for the links, I learn something new here all the time, it's wonderful.
It's really all a matter of semantics! Different folks will define things differently. I think the term underpainting is usually applied when that first layer is predominantly different than the layers that follow. (Well, that's how I define it!) This can be because the colors are different than the final colors (as in the complementary underpainting discussed), or when you do the first layer in a monochrome or a gray (usually an oil painting technique) before you add colors later, or when the application or material is different - such as a watercolor underpainting, or using a water or alcohol wash.
The big advantage to using a water or alcohol wash is that it creates an underpainting layer without pastel dust. Because there is no loose dust, the next layer of pastel that is applied will not blend with the underpainting and the colors remain true. This can be especially important when using a complementery underpainting and you want both the underpainting colors and the next layer to remain vibrant. When blended together, as you probably know, complementary colors can neutralize each other.