I detect some noses in the air and feel compelled to respond.  Although
I am a neophyte in the area of mechanical music, I have used RTV, hot
glue, hide glue and wood glue for various projects, both professional
and amateur over the years, so I thought I would put my two cents worth
in.

I would think that there would be a lot of benefits of using RTV in
organ pneumatics for several reasons.  First of all, although it does
bond and seal well and has a fair amount of bond strength, it _can_ be
separated.

Two pieces of wood bonded with hot glue or hide glue become almost a
single piece; it is very difficult to separate them without soaking in
water.  Also, hide glue shrinks considerably as it cures; if the pieces
are not perfectly fitted this can leave gaps where air might leak.
RTV, on the other hand, can be used to fill and seal large gaps, or
cracks.

Another big advantage of RTV is that it is extremely stable: an RTV
glued joint will remain just as strong and airtight after many years
of humidity and temperature cycles, while we all know what happens to
a hide or wood glue joint over time.

Yes, RTV can be difficult to clean off of a wood surface, but is it
really any more difficult than hot, hide, or wood glue?  I don't think
so.  A little scraping or sanding and the piece is ready to glue again,
although I suspect that once a surface has been glued with RTV you
won't have much luck with hide or wood glue on that surface in the
future.

There is, however, the question of authenticity.  If your goal is
to restore a historic piece, preserving its historic value, then you
probably won't go anywhere near RTV.  If your goal is, however, to
rebuild a pneumatic device that is expected to work and _keep working_,
then I think that in some places RTV may be a much more practical
choice than hide or wood glue.  I don't think most people would
disagree with this, as long as we recognize the difference in
application (historical accuracy vs. practicality).

By the way, regarding the use of RTV on metal -- There are two basic
types of RTV: one contains acetic acid and has a strong vinegar smell
as it cures, the other is based on something else.  The latter material
(non-acetic acid based) is sometimes refereed to as 'electronic grade
RTV', and is almost odorless as it cures.

The main difference is their ability to bond to bare metal surfaces.
The acetic-acid-based RTV bonds extremely well to such surfaces
because, I suspect, the acid actually attacks the surface metal and
this gives it a better 'grip'.  The non-acetic acid variety bonds to
metal in the same way as it bonds to other surfaces: the more polished
the surface, the weaker the bond.

Bottom line, if you're just trying to seal a joint with one or more
metal parts, use the electronic grade RTV -- it will be easier to
remove later on.

Les Lovesee
les@netsourceinc.com