Leonardo did it, Van Gogh did it, and Picasso did it, too. All of them painted over previous works that they did not consider worth keeping. Practice makes perfect, but not all practice turns out perfectly. There’s no point in wasting a good painting surface.
Pastelist and colorist Casey Klahn takes no prisoners. He’ll paint a hundred landscapes (or florals or portraits), weed out the 25 he likes the least, wipe them off, and then paint over them to make it an even hundred again. “Dinotopia” artist James Gurney paints over his gouache or casein work in every sketchbook until he’s happy with the whole book.
Certain media aren’t conducive to reworking the surface, but almost every medium can serve as an underpainting for something else. I prefer to gesso over acrylic paintings that served their purpose but don’t need to be kept. Sometimes I don’t cover them completely, allowing the randomness of the half-hidden work to show through the white gesso.
Pastels are perfect for this sort of thing. I have often wiped off entire paintings and started the same subject over again on the same surface, with much better results. Something about the ghost image left behind allows me to move more freely the second/third/fourth time. Sometimes, the subject no longer appeals to me, so I paint something completely different over the ghost image.
In any case. Since Tuesday, I have reclaimed another six canvases that are now ready for new work. I have my eye on a few more, too, some of them big ones. Hmm…like that one there, for example…