A few weeks ago I started making my own canvas stretchers, but I was only stretching blank canvases. Today I took an unfinished painting that I had started on an unstretched piece of canvas, built a stretcher to fit the dimensions of that painting, and stretched the painting onto the stretcher with the edges lined up.
It wasn’t difficult, the painting doesn’t seem to be damaged and the edges look fine. Now I can complete the painting on the stretcher.
The process was so easy that in future I would definitely consider completing a painting on unstretched canvas and then building a stretcher for it at the end. This means that if I wanted to crop off part of the painting, I could. If the painting really didn’t work out then I could throw away the canvas without having gone to the trouble and expense of building a stretcher and stretching it.