I have used PVC-E diluted with 3 to 4 parts water to seal pneumatic
pouches.  After it dries, dust the pouch with a little talcum powder
with a small paint brush.  This prevents anything from sticking to the
surface as it remains sticky.

PVC-E is white but dries semi-transparent and it is available from
piano supply houses or through a local piano technician.

Terry Younce