 How can I keep my canvas from sagging in the middle under the weight of the art resin? If you are working on a large canvas you will need to flip it over and stuff some cardboard or some MDF in between the canvas material and the wooden stretcher bar and the reason for that is just to reinforce it so that when the resin gets poured on top There's nowhere for that fabric to dip down causing the resin to cool So what some artists do to avoid all of this is they just start working on wooden panels from the get-go in which case There's nothing to reinforce and there's no way for a dip to occur Again, if you're working on a small canvas nothing to worry about if you go bigger than about 12 by 12 in that case Then you would want to just reinforce just for precautions and to make sure that you avoid that pulling situation