Hi,  I have two Orchestrelles, the model V and W.  I restored the V in
1989, which included a complete disassembly of the pressure channels.
I too had noticed before restoration that some of the notes were
sluggish.  The nice thing about this design is the pouch board is open
and can be watch while playing.

That white stuff was all in the pouch wells and pieces of it were in
the airways.  The channel design of this thing is sorta like the wooden
tracer bars in early Wurlitzer nickelodeons, just 1000 times larger.
This white paper gasket material was also present behind the tracker
bar.

With this giant pouch board sitting on my bench (over 5 feet long)
I removed the pouches and blew out all the channels with compressed
air.  Those channels were full of loose paper.  Blow them out in both
directions.  I then sealed all the channels and pouch wells with
shellac.  Any remaining gasket paper is caught and sealed in place.
The valves were also releathered and adjusted for travel.  New pouches
were installed and sealed.

I actually sealed the channels twice.  While the shellac was still wet
I blew compressed air under low pressure through each channel to the
pouch well.  The organ still plays and sounds great even though Don
Teach still calls it a giant harmonica.

Chris Kalstone