This by definition must be done over some type of underpainting, in a system of working in layers. If the underpainting is like a base rhythm in music, then the overpainting is like the solo. The underpainting gives a context in which the paint-strokes of the overpainting become more resonant and powerful. When properly done, overpainting does not need to completely obscure the underpainting. It is precisely the interaction of the two that gives the most interesting effects.