I too have had trouble with Hydrophane.  It appeared to work for a day
or so before it started leaking just as before.  I have a Duo-Art which
has accordion expression stacks that are wrapped with pouch leather.
Some time back I notice that I could hear the air sucking through their
sides whenever one was activated.  I tried using Hydrophane but after
a few days they were hissing just as before.

I then thinned some ordinary RTV silicone sealant with some naphtha
(lighter fluid) until it had a very runny consistency.  Then, using
a small artists paint brush, I carefully painted over the leather
covering the gaps in the stacks.  It readily soaked in.  After letting
it dry, there was no sign of it ever being there but the leather
appeared completely air tight.  That was many years ago and it is still
perfectly okay.

I have just tested it again by strapping some pouch leather over a half
inch pipe and sucking it.  Not scientific, I know, but it is easy to
detect air being drawn through it.  After painting on thinned silicone
and letting it dry the difference is dramatic, and taking it off it is
just as supple as before.

Try it, it does not take long.  I think that I would prefer to use my
method over silicone grease which I would imagine would remain tacky
and attract dust.

James Black,  UK