Underpainting
"Underpainting" in a Sentence (2 examples)
He would start with a stenciled acrylic underpainting and finish by making marks with the chalk and charcoal shells.
At first they wouldn't believe him, but he gave them proof. X-ray the Adulteress, he told them, and you will find an underpainting. They did and found the underpainting van Meegeren had described.