The answer is, that I've been combining them to try to find an ideal surface to use coloured pencil on that I can then varnish and send abroad without the need for framing under glass.
With thinking Roma might be in whelp I've stopped using pastel unless she is well away from any dust, and started this.
The first experiment was applying colourfix primer to a canvas board using a small roller. I tinted the primer with Raw Umber Chromacolour liquid acrylic as I didn't want to work on white.
This proved to be far too rough for my way of working, I could not get the fine detail for fur, although the eyes came out well.
I abandoned that one as a failure.
I then got some white gesso and again using a foam roller applied it to another canvas board. This time I let it dry and then lightly sanded it down before applying another coat and resanding. The surface this time was much smoother and the coloured pencil went on easily. As I've been promising to paint my youngest son some penguins for at least four years, I used his photograph.
The penguins are coloured pencil, the background is acrylic, it will be finished later.
I decided to carry on with the experimenting and did the same thing, but this time used a black gesso. I found it to be thicker than the white, really nice to paint on and it came out a lovely rich matte black. Again I sanded and applied another coat.
This time I found it harder to get the fur strokes in coloured pencil, so I used the pencils for an undercoat and painted on top using acrylic.
I am happy with the gesso on canvas and aim to produce some smaller pieces that I can send abroad.