Just some ideas: 

I would paint the mask a basic background color, like grey or medium/light brown, and let it dry.  Or maybe leave it wet and then add some daubs of a slightly darker color and just kind of smear them together a bit.

When dry, you might want to add a few lines or 'cracks' using a fine paintbrush or a feather, in a darker colr.

And you might want to use a kind of short or stiff flat brush, dip it into another color, then draw a stick or knife across the ends of the bristles to make the paint spatter.  (I would try a few times on paper, until you discover how to get the appearance you want.)

And if you don't like how it turns out, just cover it with a base color again, and start over.

If you want to smooth it down, you might sand it with sandpaper wrapped around a smallish block of wood.  (Just fingers behind the sandpaper is virtually worthless.)

And if you need to fill some small holes and cracks, you could find some whiting (aka calcium carbonate & ground chalk), and mix it with some white or light-colored acrylic paint.  It will thicken the paint and help provide a smoother surface.

Sue

I don't know if it would work on PM  but it would be way easy if it does - there's a spray paint that had a granite look/texture to it. Unfortunately I can't remember the name but I have seen it in both craft and home improvement stores.