If you're like me and you have a bunch of finished paintings and drawings you know you don't want to keep, here are a few suggestions on how to reuse and recycle those painting surfaces instead of just throwing them out in the trash.
1. If you're starting with an acrylic painting all you need to do is paint over it with white acrylic paint. You may need a couple or more coats of it depending on how opaque you want the white to be.
If you want to paint with oils on top of an acrylic painting you can substitute the white acrylic paint with gesso instead.
2. If you have an oil painting you want to paint over, you'll need to use white oil paint to paint over it. You may want to use a fast-drying medium like Liquin to speed up the drying process.
Note: You never want to paint over oils with acrylics. It's a bad idea.
3. The back of a canvas board or panel can be used to tape your paper to and use as a backing for painting or drawing on an easel.
4. If you have a canvas you want to reuse, you can of course paint over the used canvas or you could simply remove the staples from the back and discard the used canvas. Then you can stretch a new piece of canvas over the frame.
Stretching a Canvas for Beginners (by Michael Schmitt) https://youtu.be/FZdr2rgyD1g
5. Once you've used up a sketchbook you can take the back of it and use it as a painting surface. These are usually made of chipboard, which you can find in large sheets in most art supply stores. Again, you'll need to prime the surface if you want to paint with oils but you don't have to if you'll be using acrylics.
6. Finally, (this is probably the least archival method of all so I wouldn't recommend that you use it for commissions or any kind of important project as it will not age well) you can reuse scrap paper by either painting over it with acrylics or priming it with gesso to use with oil paints.
I learned about this method when I was an art student. I often use gessoed paper for studies and experimenting with oil paints.