With something as heavy as cotton duck, try using a wet pressing cloth instead of rewetting the canvas itself. Wet something like a dishtowel, wring it out a bit so it doesn't drip, lay it on the canvas and steam press until the dish towel is dry. Yes, it will take some time, but it should work better than a soggy piece of cotton duck. Most of the suggestions have been for quilting weight cottons and won't work on your canvas. Luck!