I've never used rubber cement as a sealer -- well, maybe once, but it
probably wasn't the right brand.  They say that only certain brands
hold up well, and all need dusting with talcum powder to relieve the
natural tackiness that makes rubber cement good for what it is normally
used for.

What I have been using lately to seal porous goods like thin kangaroo
skin is flowable silicone.  You can look in the MMD archives for
recommendations on brands.  Conrad's "whilst" suggests that he is in
the UK, so perhaps ordering via the Internet is best.  Even though the
silicone _is_ flowable, and much thinner than the regular silicone caulk
you buy in hardware stores, I find I still have to thin it with naphtha
to a thinness appropriate for the material it is being applied to.

The silicone cures in a few hours and is not tacky when cured -- and it
does seal admirably.

Matthew Caulfield (Irondequoit, N.Y.)