In answer to the question as to what is the best pneumatic covering
material available today,  I would like to share my own experience.

Back in 1977, I rebuilt my Steinway grand Duo Art XR stack with Bilon,
which I purchased from Player Piano Company.  For those people who
don't know, this material was made up of nylon sandwiched between two
thin coats of polyurethane.  I also purchased  PPCo's "Plastic Glue"
which was designed to cement the Bilon.

At that time, many rebuilders told me that I was taking a big gamble
because the Bilon would deteriorate and the plastic glue would not
hold.  All I can say is that now, 26 years later, the Bilon pneumatics
on this piano look and play as good as new, and we play this piano
constantly.

The reason I bring this up, is that, contrary to even my own
expectations, the Bilon, thus far has seemed to outperform even the
very best of rubberized pneumatic cloths (which I have also used in
restorations since 1977).   While I don't know the answer to what is
the best available pneumatic recovering material today,  I can state
with certainty that the Bilon [which I used] has provided excellent
performance for at least the past 26 years.

Barry Dennis