Hello,  The sponge I recommended is polyethylene, closed cell double
skin and will definitely stand up.  It is very, very long lived.  Even
relatively low density PE lasts a long time if not exposed to sunlight.

The adhesive is acrylic which is also very long lived.  Do not use
urethane or any other rubber that is not known to be long lasting.
Silicone would also work very well but is much more expensive and
difficult to cut.

For someone willing to take the time to cut it carefully it is probably
the best possible material.  It is very elastic and difficult to cut
accurately although ha very sharp knife and care will do a good job.
It is very stable and will not decompose and migrate into wood.  If
self adhesive material is used, it should have an acrylic or silicone
adhesive, not latex or any other rubber.  If not self adhesive, silicone
will be difficult to bond.

For someone concerned with removal, an intermediate sheet of paper
could be used between the acrylic or silicone adhesive and the valve
and could be glued with hide glue.  This would be the ideal seal but
would be a lot more work.

Another option would be to glue a strip of paper to the deck board and
adhere a strip of self adhesive silicone sponge to that and cut the
holes in place.  I would only do this with silicone as it is the only
material that would last well enough to commit to having it stuck on
the deck boards.  No dies needed this way.  I think I'll try this the
next time I do an Ampico.

 [ I asked Spencer if product standards exist and if he could recommend
 [ a maker and supplier.  -- Robbie

I don't know if there are any standards that apply to the material
that I have used.  I buy polyethylene and silicone from MSC Industrial
supply and I am sure that their sources vary.  They do give basic
specifications but not anything elaborate such as MIL spec except for
certain materials.  The poly is a cheap replaceable material and is
nothing I would consider for difficult to replace gaskets.

I have never seen bad silicone sponge but I have not gotten it from
unreliable sources.  That is why I recommended it for use on the decks
instead of poly.  I don't know if there is any bad acrylic adhesive
made but I have never had problem with any that I have used.  I would
not be surprised if suppliers such as MSC or McMaster Carr could
provide more specs that they list in the catalog for batches of
material that they have on hand.

I am also sure that it would be possible to get better and more
consistent quality from a specialty supplier such as Pacific Felt,
but you would have to buy large quantities.  They are converters and
can apply any adhesive to any material so you could specify 3M acrylic
in a variety or grades, for example.

I think, however, that just buying a well chosen material from a
reliable supplier would give better results than using something like
garment leather or a composite cork that was not made for a similar
application.

When I use a substitute material I do quite a bit of testing of the
mechanical properties and only use materials that I know to be stable
and durable.  I consider things like joint movement and mechanical
loading.  Before I used polyethylene I compared the properties of many
plastic and rubber foams and found polyethylene and silicone to be the
only commonly available materials that had suitable properties.

These materials might not be suitable for people who do not consider
things such as final joint thickness and what it might do to critical
dimensions.  For the Ampico unit valves this is not a problem, which
is why I did not hesitate to suggest it, but for other applications,
considerations and adjustments need to be made.

Best regards,
Spencer Chase