You can smooth a paper mache mask by applying a layer of Spackle (available at hardware stores) or a layer of acrylic modeling paste (available at art supply stores.) You can sand it down, apply another layer and sand it again for a very smooth finish. I hate to sand so I apply a layer of paste, then I dip my finger in water and smooth out the paste with my wet finger or with a wet, soft paintbrush.
-Alyssa
www.ravenwoodmasks.com