Phil Jamison wrote [in 1606xx MMDigest], referring specifically
to pouches:

> I never did get an answer to my basic question, "What leathers
> need to be sealed,"

The answer to this is "all leathers" if you expect them to be more air
tight.  Even the red Robert Morton leather that I tested (which I have
heard is extremely tight) proved this to be wrong -- it was much more
porous than what I had expected.

In going over my pouch samples that I sealed back in 2000

  http://www.mmdigest.com/Archives/Digests/200007/2000.07.27.09.html 
  Dow Corning #733 as Leather Pouch Sealant

not much has changed.  The one thing I noticed was that the sample that
was sealed with the Dow 111 grease was still sealing much better than
the mink oil or the unsealed side but was still far from being air
tight.

I still think the best is the RTV type used for bathroom seal, thinned
with naphtha.  Back in 2000 I used it on some of the thinnest/leakiest
leathers and it sealed completely.  Going back to these samples today
(16 years later) revealed the same result of being completely air
tight.

Last time I sealed pouches I poured the remaining thinned mixture
into a plastic dish at about 1/8" thick.  It cured and I pulled it off
the plastic.  The RTV has a thickness of 0.025" thick and is still
extremely flexible after 6 months.  You can stretch it in your fingers
without it tearing, which is a good sign that it will not open up when
closing the open pores in the leather.

Something here should be said about the apparent stiffness after
sealing.  When I initially checked how sealing affected the leather,
I only did this as a sheet; 1/2 was treated and the other not.  And
I reported that I couldn't tell the difference between the RTV and
untreated area of the leather.  Doing this same test 16 years later
on the same samples reveals the same conclusion.

RTV is still my choice for best overall flexibility and air tightness.
If you can detect a stiffness difference between the two samples on
a flat sheet then the sealer is pretty bad.

If you glue the leather into a 1" pouch well then seal it, you will
notice now the pouch raises differently than the unsealed pouch.  This
I have noticed when sealing with thinned RTV bathroom caulk.  But my
guess is it will react the same with all the other sealer's except for
the Dow Corning 111 silicone grease used for sealing bell jars to metal
plates.  I still suspect this slight difference is so small that it
won't affect the responsiveness of the valve, even though I haven't
tested this.

Pete Knobloch
Tempe, Arizona