Long term MMD readers may recall that I am a much-demonized
enthusiast of silicone adhesives and sealants, having had extensive
experience of them in the diving industry and experimented with and
used them on player pianos over the years.  However, I have not --
so far -- found any silicone compound which renders leather pouches
as flexible as Hydrophane, or is as convenient to apply, especially
to those in an assembled stack.

Silicone compounds seal leather well but the 'oils' are no more
effective than Hydrophane and pastes, which contain silica powder
fillers, and self-vulcanising RTV rubbers are inherently unlikely
to be as supple and flexible.

I have just checked the pouches of a Steinway O Duo-Art which were
Hydrophaned about 25 years ago and redone last year, and found them
effectively airtight, so perhaps Stephen Powell's test is not
representative.

When a pouch is at rest, even though the stack is under suction,
there is no pressure differential across the leather, and there is
only quite low average air pressure on the underside of the pouch for
the relatively small percentage of time a note is held on.  Even if
a tiny amount of Hydrophane exuded from the pouch during this time,
it would doubtless be reabsorbed when the pouch returned to rest.

Hydrophane works by swelling and lubricating the leather fibres.
A couple of generous applications are invariably required to saturate
a pouch completely.  Once it has done so, to lose its effectiveness
it would have to be displaced, yet I have never found any which leaked
from a pouch and pooled in the well or migrated.

One point: no amount of Hydrophane will seal leather whose pores are
excessively large, and even the best obtainable pouch leather today
is frequently sub-standard.  There is also evidence that so-called
pneumatic pouch leather, which is not actually airtight, will not
absorb Hydrophane effectively because of some treatment.  In my
experience high quality original pouches treated with Hydrophane are
usually a better bet than replacements.

The Steinway in the YouTube clip had its pouches Hydrophaned maybe
30 years back.  They're now 90 years old, show no signs of any
deterioration whatsoever and are supple and airtight.  If not for
Hydrophane they would surely have disintegrated by now like so many
we find.

Patrick Handscombe
Wivenhoe, Essex, UK