Hi Dell, I have never found any pouch leather that is 100% leak free.
I have tried using mink oil (which is really a white paste) which does
help but doesn't seal that well.  I have heard that it loses its seal
over time due to evaporation but have never tested for this.  I have
found that it has about the same effectiveness as the thinned Dow 111
which doesn't seal 100% either.  A pipe organ system is not as fussy if
the leather leaks or not.  There might be some delay but it will never
be noticed by the organist.

> To see if [sealing] is required just put the leather against
> a 1-1/4" pipe or nipple and suck.

It helps if you make a Plexiglas fixture for testing your leather.
My tester has an opening for the leather of 1 inch, with a small
tracker bar nipple on the bottom of the well.  I put a small piece
of tracker bar tubing on the nipple of the tester and sandwich the
leather between the tester and another piece of Plexiglas with a hole
in the top.  Applying a vacuum with your mouth and feeling how much
air passes through the small opening of the hose will give you a
comparison between the different samples.

I take a leather sample and mark it with a pen.  I label one side
with SEALED and the other with UNSEALED and then check both sides to
see if they have the same leakage properties.  Now I use my favorite
sealer to be tested, let it dry and then retest and compare both sides
to see how well it seals.

If you put a date on the leather sample with the type of sealer used
and stash it away for a few years, you can take it out again and use
it to compare other sealers or leathers that you buy in the future.
You might be surprised.

Pete Knobloch