|Thunder Mountain sketch, 9x12, pastel|
Sometimes I get questions about reusing canvas. My short answer for this is, Don't.
I think all of us have a collection of old paintings stashed away somewhere, paintings that should be thrown out. Because of the cost of materials, it's easy to wish that these could be recycled in some way. The worst thing you can do is to brush on a fresh coat of white paint and paint over that. Over time, the new paint will become more transparent, revealing the old painting. But there's also a good chance that the overpainting, because of differences in paint layer thickness and oil content, will develop problems such as cracking and flaking. Even if you think you'll be doing nothing but quick, throwaway sketches, don't do it. Never assume that a quick sketch won't turn out to be something valuable. Always use archival materials in an archival manner.
If you really do need to reuse stretched canvas, one thing you can do is take it off the stretchers and turn it over, exposing fresh canvas to your brush. This you can treat just like raw canvas - size it, and then apply either oil ground or acrylic gesso. Many old paintings, hundreds of years old, have paintings on the reverse side. Of course, if you paint on panels, you can simply flip those over and paint on the backs. (I'm talking about wood or hardboard panels; this won't work for those cheap cardboard ones from Frederix.)
Pastel on paper is a different story. I use Wallis sanded paper, which is remarkably durable and eminently reusable. I can take my "starts" and unsatisfactory paintings to the kitchen sink and wash off the pastel. Some of the pastel stains the paper, but the stained paper, once dry, makes for a good surface to do more paintings on. If the paper doesn't dry completely flat, I cut it up into smaller pieces for sketches. (The pastel at the top of this post is purely meant for your pleasure and not to illustrate a point; it wasn't made on re-used paper, nor will I wash this one out! It's a sketch I did the other week during one of the Paint Sedona workshops.)
So if you don't reuse canvas or paper, what should you do? Make a bonfire. Slash them with a razor. Paint a black X on them. Get them out of your studio at any cost, or these zombie paintings will haunt you forever!