It can also refer to later paint added by restorers, or an artist or dealer wishing to "improve" or update an old image—a very common practice in the past.
The underpainting gives a context in which the paint-strokes of the overpainting become more resonant and powerful.
Some of the most spectacular results can be seen in the work of Jan van Eyck.
It can be difficult to distinguish overpainting from underpainting in finished historical artworks in the absence of scientific tests.
X-rays are often used to examine paintings because they allow the conservation technician to see what is hidden beneath a surface without having to damage it, depending on the materials used.