I paint in oil, on copper panels that I make myself. Copper is a wonderful support, and cross-links with oil paint to form a very strong bond. In addition, this chemical reaction keeps the paint looking very fresh. Of course, there’s a downside which in this case is the expense of copper sheet. Still, I think it is worth the effort to produce archival works.
Lately I’ve been making panels primed with lead, using either Williamsburg or Natural Pigments. It’s a different surface to begin on, and has some advantages over copper. The disadvantage is that panels take longer to cure, which means planning ahead. But since copper only comes in three-foot wide sheets, working larger means changing primers. Not a big deal, once I figured out how to get the surface nice and smooth.
90% of the paint I use is Williamsburg, and the remainder is either Old Holland or Blockx, all high quality paints. I use very little medium, usually Natural Pigments low-viscosity bodied oil. Their extra-high-viscosity stand oil is great for certain effects, and mixed with whiting makes a great putty medium. Sometimes I use James Groves’ Copal Gelling Varnish as well, but 99% of my paint is dependent on brush work and color mixing, not medium. As a matter of fact, I think that’s the secret medium of the old masters, who, by the way, did not all live in the same city, buy the same materials or paint in the same way.
I’m a big believer in simple working methods, and dependable, proven materials. I avoid hygroscopic substrates, all unnecessary paint additions, retouch varnish and driers whenever possible.