I’ve tried to solve a “warping problem” with my collage surfaces. I’ve collaged on many different surfaces and found that a cradled board is the only surface (that I’ve used so far) that doesn’t warp.
I talked to collage artist Jonathan Talbot who was wonderfully helpful, and full of collage information and tips. He suggested a few products, one being Lumalight, which is a product sold to sign companies. That makes so much sense to me. Sign companies must need to use products that won’t warp.