Darrell Clarke asks about duo-art accordion covering material.
I think that medium weight pouch leather is the only material to use.
It is supple and is quiet when it moves. Cotton cloth or Bilon makes 
annoying crinkling noises which can be disturbing at low playing levels.

To eliminate stretching and make the fitting of the leather easier,
I glue on three narrow strips of thin double cotton cloth where the
screws for the adjustment blocks will go.  After they have dried and are
trimmed, I locate the screw holes and awl them out. This makes it easier
to locate the holes through the leather. Since the boards are now spaced
it is easier to put the leather on and also not as critical since the
cloth holds the spacing and the leather is only a seal.  Silicone vacuum
grease as a seal finishes the job.

For setting travel I use small gauge blocks make out of standard
thicknesses of aluminum. These together with some cut off ignition
wrenches make it easy to adjust the screws with the expression box in
place when necessary.  Of course the initial setup is easier to do out
of the piano. I have set accordions this way and checked the travel
under pneumatic control as a check and they agree if some care is taken
with the gauge blocks.  It is just like setting valves on an engine with
mechanical tappets.  Some feel is required to get repeatable results.
If any screws are messed up or crooked these can make it difficult, but
they should be fixed anyway.

Spencer Chase