(Repeated post, edited, from January 2014.)  Player enthusiasts of
riper years here in the UK may recall sitting, talking long into the
night, at the British Piano Museum in the 1970s with the two Palmer
brothers, whose careers were spent at the Aeolian and successor APPA
factories at Hayes.

The Palmers were unequivocally clear: Aeolian used thin rubber "cement"
on its pouches (and leather-covered Duo-Art accordions).  And indeed
here we still often come across unrestored Aeolian instruments where
evidence of the talced rubber coating can be easily detected.

And a section in the Ampico Inspectors' Instruction Book of 1919
entitled 'How to Rubberize a Pouch' details the use of rubber cement,
including sucking it into the leather by mouth and afterwards applying
talc...

In a player piano action each pouch must produce sufficient force to
accelerate the mass of its valve fast enough for perfect repetition and
to seal fully even at the tiny suction differential (about 3" water
gauge) of softest playing.  And, so that notes occur at the correct
relative moments, valve transit times and thus bleed rates must be
predictable and consistent.  The only way to ensure this is for every
pouch to be effectively airtight.

For various reasons very thin strong tan leather has been the most
usual material of choice for pouches for a century and more, but
untreated it is inherently and inconsistently porous, and therefore
must be sealed.  Though treated, even so-called pneumatic pouch leather
from some organ suppliers is apparently not effectively airtight at
player stack suction levels.

For Pianolas - not 'pumpers'! - and reproducing pianos to play
pianissimo and reproduce correctly, especially cross-valve Duo-Arts, it
is vital to achieve pouches which are airtight but immensely supple and
sensitive.  I have yet to hear any instrument rebuilt without sealed
pouches which works properly.

Rubber cement, such as Aeolian used, is natural (tree) latex dissolved
in naphthalene (benzene).  The solvent wicks the latex polymers into the
leather sufficiently to make an inseparable coating and flashes off
quickly without evaporating any of the oils introduced after tanning.
At the correct strength, a light application of the solution will
render the leather airtight without materially stiffening it, since
natural latex (polyisoprene) is still one of the softest, most elastic
substances known.

For those who wish to use rubber cement, the traditional bicycle
puncture repair liquid adhesive, such as Weldtite, is just right, and
despite health and safety attacks is fortunately still available in
the UK.  Beware the newer styrene-based substitutes which dry stiffer.
Natural latex solution and water-based latex adhesives such as Copydex
are not thin enough, do not wick into leather sufficiently and dry
somewhat stiffer unless modified.

Though it lasts a surprisingly long time in the dark, latex, being
a natural un-vulcanized polymer, eventually perishes and becomes porous.
Long-lasting self-vulcanizing synthetic Neoprene and Hypalon solutions
are available, but generally cure too stiff at about 60 Shore A
hardness.  Virtually imperishable RTV silicone (polydimethylsioxane)
adhesive/sealants which cure to a very soft and elastic 20 Shore A work
well, but the standard pastes are not as easy to apply as consistently
as the liquid versions and may render some pouches too stiff.
Attaching lifter discs to pouches treated with RTV silicone requires
judicious use of the same adhesive.

Collodion, nitrocellulose in ether and alcohol, will wick into leather
and make it airtight.  But even the flexible acetate formulation is not
elastic, dries far too stiff and soon cracks along stress lines when
flexed repeatedly.

Non-drying silicone compounds work well, but some unthinned greases
make pouches too stiff, and light silicone oils gradually disperse or
evaporate.  Silicone compounds are inert and do not weaken or rot
leather.  It is all but impossible to attach anything reliably to
pouches treated with these compounds.

Traditional neatsfoot oils soften leather and make it more supple
but like the similar oils applied after tanning essentially do not make
it airtight.  Formulations are worryingly perverse.  Some dry out and
some eventually rot leather, and those that contain waxes eventually
stiffen, especially at low temperatures.  Indeed the significant
problem with most pouch sealants is that they become stiffer at lower
temperatures and impair soft repetition.

Some 40 years ago after much experiment, I discovered a far superior
compound to any other sealant: Hydrophane Leather Dressing, made in the
UK, which renders pouches effectively completely airtight yet totally
supple and sensitive.  Importantly, it does not stiffen even at low
temperatures, and is effective for at least 25 years plus, after which
it can be simply re-applied.

This liquid does not rot or weaken leather and does not soften or
weaken any glues.  It is easy to apply cleanly with a small artist's
brush to new or old pouches, even those previously treated, without
stiffening them, and often without disassembling a stack.  I recommend
it to everyone.  Many Player Piano Group members and rebuilders around
the world can vouch how effective Hydrophane is.

http://www.battles.co.uk/products/001/equine/010/hydrophane/120/leather-care/5449/hydrophane-leather-dressing/ 

Here's an exemplary 1926 Steinway O Duo-Art, all of whose 90-year-old
pouches have been Hydrophaned and operate perfectly:

http://www.youtube.com/watch?v=0VsnBFELg-Y 

Patrick Handscombe
Wivenhoe, Essex, UK