hook,
I've found if I put on a second and third coat of TB III (TB II works too, but those are the only ones I have personal experience with) I get a pretty smooth, translucent finish! As Hrothgar says, it'll take acrylic fine (see green pic below). I've also smoothed an acrylic gesso paste onto the rough glued linen surface and smoothed that out by hand, and maybe a little light sanding. It fills in the texture of the linen/glue nicely and leaves a matte finish for painting (see yellow pic below). Both work well, in my experience.
Frode