Yes, Gary, acrylic primer has more tooth from the inert solids it uses, such as marble dust, that gives it a porous quality. In fact, Golden Paints recommends that for oil painters who want to use their primer they first size the surface with acrylic medium, such as their GAC100, to resist the oil that would penetrate through the porous primer and possibly discolor the substrate.
http://www.goldenpaints.com/technica...ix_priming.php
I've tried Yupo, and it's okay, but a bit too "plasticy" for my taste. The Multimedia Artboard I like better, since it actually uses paper. It can be literally broken if you bend it too far, but it's otherwise quite sturdy. Even 300# paper can bend under the shrinking of acrylics, but it's possible to mount it afterwards and straighten it out.
Those Constable paintings I saw on paper didn't seem to have received any obvious signs of treatment, since it was rather thin paint directly on paper, with some unpainted areas. I doubt Constable mounted them himself, although that's possible. Most were glued to stretched canvas.
Recently, by the way, I saw an exhibit of 17th century paintings here in Austin, and there was one that read "oil on paper mounted to panel" from the early 1600s that surprised me. It seemed to be in excellent condition (considering its age,) although there was no exposed paper, and I'm sure it's received loving care over time.