With all the recent discussion of Duo-Art accordion pneumatics,
one thing might have been overlooked unless I missed it.  There is a
way to minimize stretch over time without covering with two materials.
Pouch leather is quiet and correct for appearance but it does stretch
over time and (especially if it is not perfectly flat leather to begin
with) can be a little difficult to get exactly right.

I use pouch leather and apply it with spacer blocks very carefully.
Then I use thin pneumatic cloth in narrow strips, just under the
adjustment blocks to even it up and prevent stretching.  Glue the
strips with a small dot of PVCE glue to the leather between each of the
blocks so it doesn't pop out and get caught under the adjusting screws.
I have accordions that have not needed adjusting after many years of
use.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/