Bob Ross Liquid Basecoats are essential to the Bob Ross Wet-on-Wet Technique®.
These basecoats (Liquid White/Black/Clear) allow us to actually blend and mix colors right on the canvas rather than working ourselves to death on the palette. The Liquid White/Black/Clear can also be used to thin other colors for application over thicker paints.
The idea that a thin paint will stick to a thick paint is the basis for this entire technique.