As a further note about printing on curved surfaces, I would add that the
surface of something like a ping pong ball needs to be preped in order to
bind the emulsion to it.  Light sanding followed by a coat of gesso worked
when I had to print photos on ping pong balls some years ago using Rockland
Colloid's liquid emulsion.

The emulsion had to be melted in a water bath and coated and dried (under
safelight illumination) before being exposed.  The emulsion is essentially
the same as a single contrast photographic paper emulsion and id processed
accordingly.
______________________________________
Tom Gore,          Advanced Imaging Laboratory
Biology Department,        University of Victoria
Box 3020,  Victoria,  B.C.      Canada  V8W 3N5
vox  (250) 721-7134             fax  (250) 721-7120