What do you mean by smoothing? Do you mean the surface texture, or hiding the cranial cut?
In the case of the scarecrow, I started with a 2-piece skull from ACC, that doesn't have a cut. The surface texture was just the natural end result of painting it with acrylics and then clear-coating it.
Corpisying a Bucky I can hide the cut, but when I want a bare bones look to my Bucky I use auto body putty and sand it down.
However, the color differences between the putty and Bucky are somewhat noticable even after staining.
I am interested in how others here hide the cut.