I have two paintings that I did not stretch tight enough on stretcher bars and after getting them framed, tightening up my keys, and everything else I could think of, the slight sag was still bothering me. I had done the water trick on cotton canvas but never linen before today and I have to say, it is a miracle. My linen canvases are as tight as a drum now - I was a tad worried about water seeping through to the frame or just all together ruining it. It is thanks to this forum that I learnt this trick. I just poured water on the back and rubbed it with hand into the fibers - within 2 minutes, it was all perfect.