Try heavy gel medium or archival glue. Put lots of weight on the panel once you apply the canvas to the panel, for at least 24 hours. It should do the job. The gesso is probably too much, so it warps the panel.
Try heavy gel medium or archival glue. Put lots of weight on the panel once you apply the canvas to the panel, for at least 24 hours. It should do the job. The gesso is probably too much, so it warps the panel.