[ Ref: 060308 MMDigest, Use Hot Glue and Shellac ...

When I rebuild an instrument I aim to achieve at least its original
performance, and preferably the best its mechanism is capable of.
Therefore I use the best materials and techniques.  Note that 'best'
here always precludes inappropriate and irreversible changes.  So, for
instance, I ensure that all the adhesives I use are easily removable.

The idea that so-called traditional materials are always best simply
won't wash.  The originals weren't necessarily the most appropriate or
of good quality anyway -- intentionally or unintentionally.  Despite
natural materials being inherently variable, testing them clearly
wasn't exact in the past and is no better today when traditional
materials aren't mainstream.  Now, virtually none are made using
original methods, since many processes and chemicals have been banned.
Thus it's impossible to find leather the same as original, and
difficult to source suitable skins of consistent thickness and quality
whose durability is certain.

Similarly, traditional techniques were not always ideal and are anyway
often difficult to replicate.  Many stacks and expression boxes were
put together with hot glue because they were intended never to be
rebuilt, and in truth most restorers find them hard to disassemble
without some damage.  So it's irresponsible, when there are safer
approaches, to reassemble them using hot glue as this promises more
damage next time when the instrument is even older.  And if hot
cupboards and cauls aren't used, the proper hot gluing technique isn't
actually replicated.  I've seen too many immaculate rebuilds where the
chilled glue has gone crystalline and pneumatics are falling off, or
aren't sealed airtight to the decks despite liberal glue fillets.

Therefore, rather than cling to questionable materials and techniques,
the wisest course is to adopt whatever are fittest for purpose.

So, what, for instance, is the fittest adhesive for sticking pneumatics
to decks?  Ideally we want one which is economic, easily manipulated,
clean and non-toxic, allows parts to be positioned slightly yet grabs
quite quickly, requires no clamping, provides a strong but airtight
bond which can be easily parted if necessary, is easily removed and
causes no permanent changes to wood.

The fact is that RTV silicone adhesive fulfills all these requirements
and more: it provides a strong yet separable airtight bond without the
need for an intermediate gasket.  (Engineering-quality double-sided
adhesive tape may have similar attributes, but is less forgiving to
position and provides less certain sealing.)  In addition RTV actually
penetrates wood less than hot glue and is easily peeled off, after
which a very light sanding will, as normal, render the surface ready
for any glue.  But in fact RTV and double-sided tape bond best to a
smooth surface, so it's better to prime boards with a solution of water
soluble PVA, _not_ PVC-E!, which is cleaner, less toxic, more ecological,
more effective and more reliable than shellac.

The idea that PVA and RTV are new, untried and may not last is
simply wrong.  They've been in use for at least 45 years and show no
signs of deterioration even in adverse conditions.  Many 1960s jetliner
windscreens are still held firmly in place with the original RTV, and
as I said before, the most valuable oil paintings are invariably relined
using PVA.  In fact no restorer would dream of using traditional glue,
knowing the irreparable damage it caused to Old Masters in the past.

So, not least because instruments restored only with traditional
materials and techniques rarely perform as well, I'll follow the
original manufacturers, who adopted new-fangled rubber cloth: I'll use
modern materials and techniques where most appropriate.  There's no
doubt that employed correctly they'll perform and last better than
traditional ones, and be easier to reverse later.

Patrick Handscombe
Wivenhoe, Essex, UK