In which scenario is underpainting most commonly applied?

Underpainting is primarily used as a first step in oil painting to establish values and create a tonal foundation for the artwork. This technique involves laying down a monochromatic or neutral color base, which helps define the composition, depth, and light within the painting. By doing this, artists can effectively plan their overall color scheme and ensure that the forms and values are well established before adding layers of color on top. The underpainting serves as a guide that informs subsequent paint application, enhancing the overall coherence and richness of the final piece.

In contrast, the other options do not align with the traditional use of underpainting. The final layer of a painting typically involves details and finishing touches rather than the initial groundwork that underpainting provides. Creating a preliminary sketch is more of a planning phase that occurs before any paint is applied, focusing on line rather than the tonal base that underpainting entails. While underpainting can inform techniques in watercolor, it is primarily associated with oil painting methods where the layering process is more pronounced.