One of my paintings has come back from an exhibition with a severe dent in the back of the canvas. It has almost been cut through to cause a slit. I have asked my painter friends on POL for their advice. They came back with a number of interesting suggestions.
I am going to try the one where the canvas should be doused with water and a weight applied afterwards to smooth the dent. If that fails I will try the alternative suggestion with a bandage glued to the back of the canvas to prevent further damage.