I'm an artist (isn't everybody?) so I use materials that I know something about. So-called 'art store canvas' can be excellent for canoes. Because it's made to very fine standards for painters, it tends to be well-woven, tightly woven and free of knots, bumps, etc (although there are definitely grades to look at closely). If you're thinking about it, take a sample of the canvas you're planning to replace with you if you have one; otherwise, use your tactile measuring instruments (fingers) to assess the weight you'll need. Painter's canvas can have different weights than duck canvas due to the tightness of the weave, etc. As for mildew resistance, I'd highly recommend that you add a fungicide to the canvas anyway - that way you KNOW it's been done. On the other hand, the first canoe I canvassed with artist's canvas had no fungicide added to it, and lasted at least twenty years until it was stolen. My first hope is that it lasted until approximately half-way through the thief's first trip, which leads to my second hope, which is that it was a long trip, far away from an art store.