This is a lesson on impatience.
About 2 months ago, I completed two 24"x24" textured acrylic pieces on 1/8" thick hardboard (masonite). I cut the hardboard myself, sanded them and coated each with 3 or 4 layers of gesso, sanding between coats. I fully intended to prevent warping by cradling them with strips of wood along the back edges before starting any artwork on them.
However, in an intense fit of inspiration, creativity, impatience (and lack of other available supports), I began and completed both pieces in acrylic paint and heavy gel medium BEFORE I actually got around to reinforcing them.
Even as I was doing them, I was thinking to myself, "these are gonna warp, I shoulda waited". I was right. They warped. They are now slightly concave. When laying flat with the artwork facing up, the edges stick up about 1/4".
I think I may be able to mount them in frames to fix/hide the warping, but I thought I better ask one of you knowledgable folks in case someone has a better suggestion.
One of the pieces is shown below. Sorry for the crappy photo.