Beam recommends illustration board, but I used mat board since I have a lot on hand. After the medium/wall patch dried, I applied gesso so I would have a nice surface for painting. Below are a few of my textured boards, ready to paint.
Scratches with a bamboo skewer
The medium was squeegeed through a piece of heavy lace. Can't really see the lace pattern, but it has an interesting texture.
Here I used a spring whisk, part of a plastic ceiling grid and a skewer.
This is pretty much like using the sgraffito technique on flour paste, so I felt right at home creating these backgrounds. Then I laid the first wash on one of them. That takes me back to my watercolor days. I love the soft effect even more with the textured background. And when I paint over it, those colors won't bleed into the 2nd layer.
It's a slow going process, since I only have a short amount of time to devote to this right now. But I like working in stages like this. And I know they don't look like much, but I am having so much fun doing this. The neat thing is that I don't really care what the end result is - I'm just enjoying the process!