Thank you for sharing this! I'll be trying it! At the moment, I'm making dozens of "book spines" for wall decorations in our church, so this will come in handy to cover the cardboard armatures! (For those who speak German, the joint compound is either Fügenmasse or Spachtelmasse.)
DAS air dry clay doesn't ever crack for me, especially if you go ahead and gently paint it (slows the drying so no cracks,) and glue it to your project right after or before you paint it, so it won't really shrink either ...