Plaster hardeneds as it gets older, but is still subjected to chipping if handled roughly. Painting might help if thick enough, but don't count on it. If you are looking for a level of protection, several good layers of varnish would help.
Probably a little late to suggest this, but I would think that it would be easier to start with a book. Take an older hardcover book, like one you can find at Goodwill and such, and bind the pages together with glue, staples, etc. Then you could put the face on the cover using a paper-clay product.
Another thought occurred to me is you might had a layer of latex to the surface. That would protect it some and a tactile element closer to skin. The latex can be painted over as well.