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MMD > Archives > July 2000 > 2000.07.27 > 09Prev  Next

Dow Corning #733 as Leather Pouch Sealant
By Pete Knobloch

I have received mail asking the question "What is RTV?"

Larry Mayo also asked about which type of RTV (alcohol base, or acetic
acid based stuff) did I use.  My answer to this is that I don't know
about these different types.  This may be a concern that I should know

Reading the MMD Archives got me interested in trying the Dow Corning
#733 100% Silicone Rubber Sealant.  I know that many restorers say that
there is no place for this type of product in the player piano.  They
are talking from experience when somebody used this sealer to mount
hose nipples, or to close air leaks between two wood surfaces by
smearing a thick layer between or around the wood.

I have also had to work with someone's misuse of this product.  I was
skeptical about using it but started running some tests with it anyway.
The more testing I do, the better I like it.  I haven't used it in a
player yet but this is the one I like the best so far with my testing.

The MMD Archives has information about what RTV stands for.  Look it
up.  ["Room Temperature Vulcanizing"]  I looked for the #733 product
and found that Grainger's had it under the part # 5E077.  When I got
the product, it was a DAP product labeled DAP 50-year silicone rubber
sealant for windows and doors.  They said it is a direct replacement
for the Dow corning #733.  The tube says it meets ASTM C-920 Class 25,
Type S, Grade NS (whatever that means).  The information on the DAP
product said "manufactured by Dow Corning and marketed by DAP".  This
is what I used for my testing.

This RTV cures in open air and can _not_ be painted.   When dry, it is
very flexible and sticks to glass and metal very well.  It feels like
soft rubber.  I have seen this product applied around bathroom tub
enclosures and it is still soft after 25 years.

I thinned the RTV using naphtha.  The best thickness I found is
compared to warm maple syrup.  Don't use it right out of the tube.
Putting it on too thick causes a slight change in stiffness.  Putting
on too thin a layer doesn't seal the pouch leather completely.  A
second layer can be added easily if this is the case.

I used a small brush to apply the mixture to the pouch leather but
using a disposable Q-Tip would probably work just as well.  Apply it
just as you would rubber cement.  The color on the top of the leather
will change, based on how much silicone is absorbed into the leather for
the first layer only.  Applying a 2nd layer isn't necessary if thinned
properly.  I found that applying a 2nd layer didn't change the color of
the leather when first applied.  I guess that the 1st layer of RTV is
now protecting the leather from absorbing the 2nd layer.

Applying the 2nd layer didn't seem to hurt the stiffness of the
leather, which I didn't understand.  I guess the layers were so thin,
and the RTV so flexible, that it doesn't mater.  It would be good to
test the stiffness using Bernt Damm's method of multiple pouches
connected to the same hose and inflating the pouch very slowly.

All of my tests showed that the RTV doesn't soak to the opposite side
of the thin pouch leather.  No color changes were seen on the opposite
side of the sealed leather.  Even after 40 days, which is the time that
I started doing these tests.  It didn't seem to change the stiffness of
the leather when thinned.

I tested for leaks by using a small plastic block valve tester which
has a 7/8 inch hole on the top and a small hose nipple on the bottom.
The leather sample is laid between the value tester and a 2nd piece of
plastic with a hole on the top.  I create a vacuum by using my mouth
on a tracker hose to see how well the leather seals.

I also ran about 5 glue-ability tests using the RTV product, which was
another concern.  One of the tests was done using a single piece of
pouch leather and applying RTV to the leather and letting it dry for
about 4 hours (1/3 untouched, 1/3 sealed 1 side, 1/3 sealed both
sides).  The leather was then glued to poplar wood using animal hide
glue and left to sit for 2 days.

The part of the leather sealed on both sides didn't glue very well,
but the other 2/3 of the leather just separated and stuck to the wood.
Conclusion was that the silicone didn't wick through the leather and
affect how the leather could be glued.

The next question I had is "Does cured dry RTV being pressed against
a wood surface affect how the wood can be glued later?"  At no time
did uncured RTV touch the wood.

Using the wood from the previous test, I then lightly scraped the area
where the dried RTV was touching the wood and reglued a clean piece of
leather to the board and let it sit for two days.  When I removed the
pouch leather from the wood for a 2nd time, it separated along the
entire surface of the leather.  Scrapping the wood with a razor blade
showed that the leather was glued equally across the entire part of the
wood.  The dried silicone on the leather from the previous test did not
affect the gluing properties of the wood.

I found that using talcum on the sealer was helpful.  The sealed
leather would stick to my plastic tester surface if not dusted.
I haven't found a product where talcum wasn't needed, egg being the
exception (which I haven't tried yet).  You want the valve lifter at
the base of the valve stem to move freely on the leather if not glued.

I think that this sealer works so well because it has the ability to
stretch with the leather when placed under tension and doesn't pull
apart from the leather when dried.  It adheres to so many surfaces.
This is probably one of the reasons why a good quality contact cement
was used for so many years.

Pete Knobloch (Tempe, AZ)

(Message sent Thu 27 Jul 2000, 18:52:39 GMT, from time zone GMT-0700.)

Key Words in Subject:  733, as, Corning, Dow, Leather, Pouch, Sealant

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