I liked the sky and clouds of a failed beach painting, so I painted out the unwanted section with gesso. I would normally paint over a section with coloured paint, but the gesso seemed to restore the light-sharing properties of white canvas. I was still able to get transparent-looking water, even though there are colour layers under the latest gesso layer.
That was easier then taking the whole canvas off the stretcher frame and stapling on new canvas!
This shows the gessoed bottom half, with frisket applied to water highlights