To answer Pete Knobloch, I've been using OSI leather conditioner
for over 35 years now and I know that product pretty well.  Let me
warn you that is not a good sealant at all for porous pneumatic
leather.  It does have a sealant effect for a short while, right after
it has been applied and while it is still wet, but after few days
curing the leather is almost as leaky as it was before the application.

If you read the OSI specifications for the product it says that it
protects the leather from being attacked by alkalis and acids that
are in the air, e.g., pollution in urban areas and sooth from votive
candles and lamps in churches.  Nothing is said about pores in the
leather.  In certain circumstances it can give few extra years of use
to aging pneumatic leather; those are the uses for which I'm still
buying the stuff.

When releathering player pianos and pipe organs I use medium pneumatic
leather everywhere that I can, to avoid too-porous thin or extra-thin,
or I use Morton-style pneumatic leather and when needed I apply naphtha
diluted rubber cement with talc -- the good old ways...

There was another compound I used many, many years ago that was very
good, too.  A recipe I got from a long-gone old retired employee of
Casavant in the early 'eighties: a dilution of leather dubbin, beeswax
and naphtha.  But nowadays I can't find the same dubbin we had in those
years.  It was a product sold by Bata Shoes, all made of mink oils, wax
and neatsfoot oil.  Alas, when Bata Shoes disappeared in the 'nineties
so did the nice dubbin they used to sell for winter boots.

Gilles Chouinard
Laval, Quebec, Canada

 [ At https://en.wikipedia.org/wiki/Dubbin 
 [ "Dubbin ... consists of natural wax, oil and tallow.  It is different
 [ from shoe polish, which is used to impart shine and colour to leather."
 [ -- Robbie