I would like to apply oil pastels to canvas. I would start by applying acrylic gesso and then several coats of GOLDEN Gessos and Grounds, specifically Absorbent Ground (White). I am wondering if I can add GOLDEN acrylic paint to Absorbent Ground. I am also wondering if there is a varnish I can use over the finished painting.
You can either mix paint into the Absorbent Ground, or do a wash of paint (Fluid Color thinned with water). The drawback of adding paint is it lowers the absorbency if you add too much. Medium pastel shades are about as far as we suggest.

We also have a product called Acrylic Ground for Pastel. It provides tooth but in a translucent film. It is not super clear, but you can see an underpainting through it. This can also be tinted.

Oil Pastels are a little trickier to varnish. Normally, one would assume MSA Varnish is ideal, but the pastels have a solvent-soluble wax in them and you risk smearing the oil pastel. If you try this product, you should probably spray light coats until you build up a film. We've tested waterbased mediums and the Polymer Varnish over some oil pastels and it seemed to work really well. Soft Gel Gloss, thinned 2:1 with water is a good place to start. Don't worry about sheen as you can always apply a matte or satin varnish afterwards.