I recently talked to Dow Corning technical department and explained to
them that I was looking for a sealant to seal leather pouches that was
very flexible and had great adhesion properties.  They recommended
using Dow Corning 832 Multi-Surface Adhesive Sealant for this purpose.

I did some testing on pouch leather and thinned the sealant down with
naphtha, about 3 naphtha to 1 sealant.  This ratio isn't scientific,
but after applying the sealant and curing for 24 hours, the area on the
leather was totally airtight and extremely flexible.  I tried to remove
the thin sealed layer from the leather and it is almost impossible
without damaging the leather.

A few years back some of us on MMD discussed using Dow 734.  This
sealant seals fine, but it really isn't also an adhesive like the Dow
832 is.  There have been ongoing concerns about the sealant not
sticking to the leather because of oils in the leather.  Dow does say
that the 832 is a multi-surface adhesive product and sticks to most
surfaces.  Not only will this thinned down sealant possibly work with
pouches, but probably it will be ideal for coating bellows where the
cloth is still okay but the rubber has deteriorated.

I'm in the process of doing more testing to make sure that the sealant
will not delaminate from the pouch leather.  Also, I tested the
adhesion of the same sealant for the purpose of holding the valve
lifting disk on to the middle of the sealed pouch (a design which some
player actions use).  It did not have the holding power that I would
expect; in larger portions of the sealant it seems to tear too easily.

If one puts a thick coat of the non-thinned 832 sealant on a piece of
pouch leather and let it cure, it does adhere to the leather, but can
be peeled back easily.  Since it is so flexible, it makes since that in
a large amount of cured sealant, it would tear.  However, in the area
that I peeled the thick layer of sealant off the leather, the leather
is completely airtight.

The sealant seems to stick much stronger in the micro pores of the
leather and at the same time it seals the pores.  This is what we are
looking for; flexibility and air tightness.  Before applying the sealant,
the pretesting of the porous pouch leather showed that it had slight
leakage when trying to blow through the leather.

Dow Corning 832 can be found in an industrial supply company such as
K.  R.  Anderson.  You won't find it in your local hardware store.
Since this information is completely empirical, I hope others would
offer comments about this.

Bing Gibbs
Cupertino, CA, USA
http://www.playerpianoforte.com