Sometimes I paint over an unsuccessful (okay: failed) painting when I feel the urge to experiment.
The canvas can't be wrecked anymore and somehow that takes the pressure off and frees me to paint in a relaxed and loose way. The paintings don't always work out but when they do it's because of this playful and low-pressure approach.
This series shows the original canvas - a smeared-off still life of some jars of jam - and it's transformation into a floral.
I find it's easier if you rotate the work from its original orientation so that there isn't an established top and bottom to the piece when you start. It's not a blank slate but it's interesting tone and colour which allows you to paint over it in a surprisingly free way.
When you do this in acrylic, it's a simple matter of just applying fresh paint, but it's different in the world of oils. You have to either paint over a very old, dry work (a year of more), or know the "leanness" of the work that's already on the canvas. I don't do this if the failed work has a full range up to quite "fat" (thick and oily) paint on it. Then I'd have to paint even fatter: in nothing but impasto paint, to follow the oil painting rule of fat over lean and avoid a painting that will crack as it dries.
In the case of this still life, I had rubbed off the majority of the paint while it was still wet and so what was left was little more than a stain on the gesso. I could safely start a new work over this though I didn't thin my paint with any solvent which would have made it too lean; I only thinned with walnut oil. The original colours are still visible throughout the work and add interesting, lively midtones to the new work. I think it's good recycling and fun to boot!