I have used Plaster of Paris fairly successfully.  the main thing to remember is that the addition of water to P of P starts the chemical reaction that causes it to harden.  As a result you have to work fast before the pulp becomes unusable. 
However if used with tissue/toilet paper types you get the most fantastic porcelain hard finish that sands well.  I tend to add no more that a heaped tablespoon full as a filler to about two good handfuls of paper pulp and paste mix.  I beat the whole thing well with a fork and mash it up then use at speed.  I use clay modelling tools for fine detailing.

If you're applying the plaster just on the surface instead of mixing it with the paper, apply it with a flat brush as quickly as you can so it can smooth out by itself.  Several thin layers may produce a smoother surface than a single heavier layer.

Sue