Celluclay question

Put some water on your finger or whatever implement you're using and it will smooth right out underneath your tool.

If you're looking for REALLY smooth though, pit a thin layer of plaster over it after it dries for a day or two.

Layer a thin coat of watered white glue over the surface....  

Smooth the surface while the Celluclay is still wet.  You've got a window of time before it begins to dry.

your best bet after you put it down is to let it dry and hit it with some high grit sand paper and just smooth it down.