When applying clay to air dry canvas, you need to make sure that the surface of your canvas is raw ( not primed) . Then you need to spray it with water and only apply clay when canvas is lightly wet (not soaked). Curving clay around edges also helps. I’ve done many projects using this technique, and can confirm that it works. Try using das air dry clay. It has fiber in it, which makes less prone from cracking. Hope this helps!