oil paint on paper
I can't find the thread this replies to (after two days offline "they" sent me 100 e-mails. But for the general info and what it's worth, I mention what they taught is in the distant past -- what do they call that, the Pleistecine period?
That is, we were in art school, and among other media, very interested in oil paints -- we learned to grind and mix our own, also the "rules" about proper treatment of surfaces to put them on. These usually required an "isolating" layer, something oilproof to protect the underlying canvas which, when the oil soaked into it would, after x number of years, rot. Also turn yellow, making the colors on it mostly yellow as well.
If I had to name those layers now, I'd get it maybe 67% right, first a coat of rabbit skin glue (isolating varnish?), then gesso (not sure you could buy it ready made, but we learned to make it), then a coat of lead white, which gave a white base and didn't yellow, and/or maybe some others -- or not. (All that before a dab of color for the "art" -- tho fortunately in those days, lead paint wasn't poisonous.)
But for quick studies, experiment, new styles, and just the heck of it, we also loved to paint in oils on paper (infinitely easier and cheaper than stretching linen, or even cotton canvas which cost $, as did the stretchers). The only rule with painting on paper was a thinned coat of shellac to protect (isolate) the paper from the yellowing/rotting effects of the oil in the oil paint.