When I'm working in a watercolor sketchbook, I don't get too worried if a sketch doesn't work out, because I can always start a new picture on top of the failed one.
This sketch of the draft horse Turk was painted over a restaurant portrait that got off to a wrong start. Beneath the horse, you can see the ghostlike yellowish shape of a man's head at center, with blue color around it. When I got home I just wet the whole surface of the paper and scrubbed out the details.
The next day I was visiting the farm, and I liked the way Turk looked in his stall. So I painted him over the portrait palimpsest. In the bargain I got a light effect that might not have occurred to me otherwise.