Working in layers

Then comes underpainting, overpainting, and finally semi-transparent glazes and varnish.

Areas not underpainted, outlining the space for a figure for example, are said to be reserved.

Working in layers has been utilized by many schools of art over many centuries.

For example, in the early 15th century Cennino D'Andrea Cennini describes how to paint in layers in the egg tempera medium.

[1] Modern techniques such as X-rays and infra-red reflectograms often enable lower layers of paint and underdrawing to be seen, and reveal pentimenti, or changes of mind by the artist in the course of work.