Lately I've been doing some sculpting with paper mache and air-dry clay. This is more of an explaination of what I do, not really a tutorial. Perhaps someone shall find this useful?
I've been using newspaper-type paper and Crayola branded air-dry clay for sculpting, because one is free and the other is relatively inexpensive. Both have limitations, however. Glue and paper pulp has a really rough texture when dried, and the clay is heavy. I experimented with putting the clay over the paper mache and figured out a way to do that which works for me. I use Elmers glue, because it was the cheapest glue at the hardware store I went to. Paper bags are also helpful.
First, I create a structure to sculpt on top of. I've used wire, cardboard, masking tape, and hot glue. I'm basically trying to do the rough shape, but a bit smaller than I want the final piece to be. Balled up paper makes a good sphere. Paper bag strips can also be helpful for this step.
I further refine the base by glueing strips of paper onto it. I cover any gaps that I don't want and paint glue on top of the paper, so it hardens when dried. I have attempted to add volume with paper strips, but I think that's best done in the next steps.
I think this step could probably be combined with the next step, but I do it anyway. I mix wet paper pulp from newspapers with craft glue. I find that the paper pulp sticks better to the base if you paint on some watered-down glue before applying. I've found that the material shrinks down a lot, so you do lose some volume from water loss here. The texture gets a lot rougher after drying.
I've found that air dry clay on it's own shrinks down and cracks on top of the paper mache, so I first do a layer that has paper pulp in it. I mix together wet paper pulp and air dry clay, and paint that onto the sculpture.
I mix the clay with some water, then paint that on the surface in order to get a smoother texture.
Using a soft brush to gently brush the (dried) clay seems to work pretty well for smoothing it out. I'm unsure how best to water proof the sculpture after this.