I recommend Pelseal for sealing of leather.  It dries completely
flexible and 100% airtight and will remain so, in my experience,
for over twenty years.

I think they supply samples, but the chemical is very expensive --
about $200 for the smallest size sold, one quart.  For our purposes
I have reason to believe that the catalyst supplied would be better
unused.

  Pelseal Technologies, LLC
  Bensalem, PA 19020
  http://www.pelseal.com/ 

Call 1-215-497-1088 and ask for Gary Hoover.

Larry Broadmoore
San Fernando, Calif.