Noun
(art) An initial layer of paint, often monochromatic, applied to a ground as a base for subsequent layers.
(art) A painting that the artist later painted over to create the final work.
Source: en.wiktionary.orgThe first coat (the underpainting ) is laid down, often painted with egg tempera or turpentine-thinned paint. Source: Internet
It does seem that Hals usually applied his underpainting very loosely: he was a virtuoso from the beginning. Source: Internet
The background inscription gives Edward's age as six, but this has been doubted after x-rays of the underpainting. Source: Internet