If you are pouring a large quantity of ArtResin epoxy resin on a big canvas, there is a risk of the weight of the resin causing the canvas material to sag in the center. To avoid this, first brush on a very thin preliminary coat of resin. This first coat will make the surface very stiff. After you have this hard base, apply a second coat to get the thickness you want. You can also support the canvas by custom cutting a piece of MDF board or cardboard and fitting it in the back of the piece, under the crossbar of the stretcher. Alternatively, you may want to work on wooden panels which will not pose the same risk of sagging.