The frotté (“rubbed”) technique involves a thin layer of oil color diluted with thinner, applied with vigorous brush work. While the palette is largely composed of earth colors, the image is actually worked up in a relatively full palette even at this early stage. This approach establishes a lean (low oil), smooth underpainting that supports subsequent layering.
Bright (short flat) natural bristle brushes work well for For the frotté technique (though David’s work precedes the development of metal ferrule flat brushes). http://www.utrechtart.com/Utrecht-Series-219-Chungking-White-Bristle-Brush–Brights-MP28322-i1005897.utrecht