Rubens remains my favorite painter to this day. I read up quite a bit on his history and studied his working methods. He begas most paintings with a streaky gray ground intended to give a degree of animation to the surface. He then used ochre, burnt umber, white, and a touch of black to establish a monochromatic underpainting that established the composition in terms of value:
Various journals and biographies state that Rubens used a medium similar to the traditional three-part oil medium of linseed oil, turpentine, and damar varnish. These paintings would then be placed in the sun to help avoid yellowing in the whites.
Rubens then applied the colors using a variety of glazes and impastos. There has been much study made of his glazing methods due to the fact that the colors remain so fresh, and the artist was able to paint far more quickly than Titian... the artist whose work he most admired. There have been suggestions that he developed a glazing medium similar to Maroger's that allowed for transparency and yet set up immediately... like a gel... holding the brushwork... and yet was able to be reworked for a reasonable period of time.
The artist began applying a full palette to the central figures. If the painting were a lesser commission, a project that needed to be quickly realized, or one that didn't fully engage him, he might have assistants and apprentices paint the secondary figures and the background.
In an important painting that needed to be rapidly completed, Rubens would employ any number of the top painters who specialized in various genre: Jan Breughel for flowers, Franz Snyders for animals, etc... The finished painting is polished with little or no underpainting showing through.
If you compare one of his finished sketches (above) with one of the finest paintings solely by Rubens" hand such as the
Judgment of Paris
you will notice the free handling in the secondary details (no highly rendered leaves on the trees, etc...) and the steaky gray and ochre underpainting can be seen in the background: in the tree trunk, the sky, etc...