One interesting technique for sandblast masking uses a
photosensitive material. The material is pressed onto the
workpiece, and the design is exposed onto the material either by
enlarger or by a contact print process. A developer solution is
then used to dissolve away the exposed portions leaving a
rubberlike mask to be used in the sandblasting process.
I have not used the material yet myself, but my brother reports
its use for very fine sandblasting at his company. This may also
be the technique used by our rock club to produce a picture of
the membership on a slab of marble.
More details can be obtained from Photobrasive in Duluth, MN
Fax is 218-628-2002.