A restoration project for Leonardo da Vinci's "Virgin of the Rocks" has revealed new details and suggest the Renaissance artist may have painted all the picture himself, instead of with his assistants as previously thought, a British gallery said Wednesday.

The 18-month conservation project involved removing much of some badly degraded varnish that was applied to the painting in the late 1940s, enabling experts to take a much closer look at the picture's brush strokes and styles, the National Gallery said.

The cleaning revealed the painting's full tonal range, especially in the darker areas, and resulted in a clearer sense of how the artist intended for space to recede through the rocky landscape, the gallery said.

It also affirmed that Leonardo likely painted the entire picture himself and intended for it to be unfinished.

The restored painting showed a range of completion from the barely sketched hand of the angel to the fully realized heads of the main figures — consistent with many of Leonardo's works. The Italian artist, said to be the "eternal perfectionist," is thought to have left his pictures unfinished because he wished to return to them later, gallery spokesman Thomas Almeroth-Williams said.

In the past, scholars believed the different levels of finish in "Virgin of the Rocks" showed that Leonardo was helped by assistants.

The painting dates from about 1491 to 1508 and is a later version of one on display in the Louvre in Paris.

The latest cleaning project followed years of scrutiny of the masterpiece.

In 2005, experts using infrared technology found two drawings hidden beneath the surface of the picture -- one design was never painted, and the second one revealed Leonardo changed his mind about the subject several times.

The painting goes back on display in the National Gallery on Wednesday.