I don't read the celluclay instructions, never have. I almost don't even put water in now, but almost entirely white school glue, only, plus the colouring.
Also, I took a tip from Manstein's Revenge; I glue some sprues around edges and all over the base, the form the celluclay over that. As it dries, it does not crack due to very low water content to begin with, and grips the sprues, thus does not lift off the edges.