I've been testing out a variety of papers for my automatic drawings. I'm not being very scientific about this, just trying things out with my style of working. I have done a large amount of them in a Canson Universal Sketchbook. It's a great sketchbook, the paper is sturdy and accepts light washes with ease. The paper cockles when I use more watercolor. With ink it's perfect.
So I taped down some pieces of Canson XL Bristol. I like this for ink too, in fact of all the papers the ink really pops off this page. It's bright white and after watercolors dry the paper is smooth and flat. It's really great but I didn't like how watercolors behaved on it's surface. It was absorbent and dried very fast.
Next up I tried another Canson paper, Montval, not or rough watercolor paper. It's one of my favorites with watercolor. I love the nooks and crannies of it's rough surface, it makes watercolors blend in amazing ways. Ink also looks great on it, but the pen tends to skip across it's surface. It's a little harder to draw on than the other papers. When it dries, it's perfectly flat.
I think what I'll end up using is a cold pressed watercolor paper. The hard part with this whole thing is that I never know what I'm in the mood for, I let these images grow organicly and the end result is that I never know if I'm going to cross hatch the hell out of an image or flood the page with watercolor, so I need something that can handle both.