Here's my favorite way to prep my bigger canvases for pouring resin on top of, and to get rid of/or prevent sagging canvas! It's perfect when you have some resin laying around that has ambered or you just don't wanna use for other projects.
Check out the video below to watch me! In the video, this particular canvas also came loose from its spline and I would have had to remove it completely to re-stretch it. If I had supported it from underneath with a piece of cardboard etc, because it's so big and the canvas loose, the weight of the resin would have probably made indentions where the edges of the cardboard ended under the canvas.
A lot of times with canvas that has sagged, or smaller canvas... you can spray the back of it with water and use your heat gun to dry and tighten it up. It may still need support when pouring resin over it, but the resin shouldn't pool into the center. However, with this particular canvas since it came loose, I added some staples to keep it in place. I attempted to tighten it with water and heat but was unsuccessful.
The trick here is to make sure the resin goes under the stretcher bars and the bigger that canvas the thicker the resin has to be. In the below video, the 24x36 canvas ended up getting 2- 24 ounces (48 ounces) to stabilize it. Happy pouring! Any questions, ask below!