There are two main types of rubber cement: solvent-based and
water-based.  The solvent-based variety is the older of the two and
is what was (and still is) used for sealing pouch leather.  The solvent
is called heptane, and it will thin the common brands of cement like
Elmer's, Carter's, etc.

You rarely will find the solvent sold where the rubber cement is sold,
but types of stores that carry it are art supply stores or picture
framing supply stores.  The usual brand of heptane is Bestine.  The
matching cement brand is called Best-Test White Rubber Paper Cement.

For sealing pouches, I mix it 50/50, or preferably thinner, and apply
multiple coats if needed.  A thin coat is better -- it won't stiffen
the pouch.  Dust the pouch with talc after applying to remove tackiness.

Make sure the work area is well-ventilated when using it -- the fumes
are pronounced and I'm sure not good for the human body.

John Runge

 [ At http://en.wikipedia.org/wiki/Heptane : "Heptane (and its many
 [ isomers) is widely applied in laboratories as a totally non-polar
 [ solvent. ... Heptane is commercially available as the rubber cement
 [ solvent "Bestine" and the outdoor stove fuel "Powerfuel" by Primus."
 [ -- Robbie