I recently bought a skin of the Columbia extra thin "Tan Pneumatic
Leather" (code CTL), and it's exactly the same thickness as the
original pouches in my Duo-Art.  Columbia also offer "Columbia
Maroon Leather" (code CML) which is identical other than its colour.
I figured that tan was a better match for an Aeolian player piano.
That prompts my observation on the ongoing 'sealing' debate...

Pouch leather is a flexible fibrous base supporting an appropriately
airtight face, and the necessary airtightness can be achieved in the
manufacturing process (the degree of airtightness needed varies between
systems).  Columbia explicitly call their factory process "sealing",
and describe it thus:

  "The skins are subjected to significant pressure which forces the
  skin's natural oils to the surface where a specific amount of heat
  is applied to cause these oils to seal the pores in the skin without
  damaging the cellular structure of the leather."

Their CTL/CML leather is therefore sealed at the factory, and so
it's questionable whether any additional sealing is needed or even
appropriate for pouches.  It's inaccurate to call it "unsealed" just
because it's not been coated with some substance by the installer!
I have absolutely no idea if any other leather supplier offers
pre-sealed pouch leather, or of the degree of additional airtightness
the process imparts.

Not all leather is factory-sealed, though, and indeed it doesn't need
to be.  Columbia's "Columbia Pneumatic Leather" (code CPL) is not
described as sealed, so I presume it's not.  Clearly many applications,
such as valve faces, just don't need to stop air flow through the face
of the leather.  If such leather is used in a situation where it needs
greater airtightness than the factory product offers then it may need
to be sealed on installation.

So, you need to understand the needs of your application, the
capabilities of the leather you buy, and match the two.  If you buy
pouch leather made with the necessary airtightness you just install it
as received!

Julian Dyer