Judging by my emails I guess I've betrayed our best known secret here
amongst UK Player Piano Group rebuilders!

Hydrophane Leather Dressing is a water thin, extremely pure, fat and
wax free dressing oil.  It rejuvenates and increases the  suppleness
of leather and zephyr skin (providing they have not crumbled and lost
their integrity).  And -- most importantly -- it effectively renders
them airtight without stiffening them, especially in cold weather.
It's probably mostly why we seem able to make our Pianolas and
reproducers here in UK play at such soft levels without missing!

It's very clean to use and can usually be applied to pouches using an
artist's brush without disassembling a stack.  Apply judiciously over
a few hours until no more is absorbed.  It doesn't affect hot glue.

It's effective for perhaps 20 years and can be reapplied in the same
way without a stiffening build-up of rubber or other airproofers.  Old
pouches treated with Hydrophane Leather Dressing often perform just as
well as modern leather replacements.

I discovered it after a search for a neatsfoot-type oil which didn't
thicken and stiffen at low temperatures.  Hydrophane are well known
to equestrians here in the UK and probably have agents in the US.  They
don't say what's in their Leather Dressing but it most likely contains
only low molecular weight polyalphaolefins (but which are high enough
not to evaporate quickly).  I and several others here have been using
it for about 30 years and swear by it.

Make sure you use the Leather Dressing and not their other products
containing lanolin and waxes:

 http://www.battles.co.uk/productlist.asp?product=Leather+Care&submit.x=74&submit.y=25 

I've been known to work up a bit of a sweat while rebuilding a stack,
but so far I've had no problems with mud...

Patrick Handscombe
Wivenhoe, Essex, UK