Sentence:  At first they wouldn't believe him, but he gave them proof. X-ray the Adulteress, he told them, and you will find an <WSD>underpainting</WSD>. They did and found the <WSD>underpainting</WSD> van Meegeren had described. 
Meaning of the word marked with <WSD> tags:  A painting that the artist later painted over to create the final work.