Sometimes when I get my canvases home from the art supply store, they are flimsy and loose. It is no fun at all to paint on a loose canvas. When the canvas is loose, I hammer little keys into slots in the corners of the back of the canvas to tighten the front painting surface.
Yesterday, when I tried to tighten a canvas this way, the little keys pushed through the support and out the side. See? Not good. So, I called the art supply store to explain, complain a little bit, and hope to get money back for something I accidentally helped destroy only because the product wasn't up to a properly stretched canvas standard.
The sweet, kind manager was understanding and very helpful, agreeing to fully reimburse or replace the faulty product, but he also taught me a new trick.
Apparently, at least with linen canvases, instead of using the keys to tighten the painting surface, one can take hot water and paint it on the back of the canvas. The hot water shrinks the canvas until it's taut. Of course! Why didn't I think of that? Duh. And why hasn't anyone mentioned this before? It would have been a most helpful trick to know. Learn from me, Grasshopper.
The discolored part of the linen is where I've already painted water on the back of the canvas. Even before it was fully dry, sure enough, the surface was tight as a drum. Perfect for painting.