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MMD > Archives > March 2011 > 2011.03.07 > 07Prev  Next

RTV Silicone Adhesive For Rebuilding a Pianola
By Paddy Handscombe

I find that I must have missed some posts on this topic last time it
arose, and failed to answer some questions raised by John Farmer and
others.  Normal RTV silicone adhesive absolutely can be removed
completely from most substrates.  We are not talking here about liquid
silicones which soak into absorbent substrates as oils do.

Normal RTV silicone adhesive/sealants are acetoxy curing.  Some
catalyst-cured RTV silicone rubbers which are applied to difficult
engineering substrates like glass with primer-activators are much more
difficult to remove.  Dow Corning and now other companies make a huge
number of RTV silicone adhesives/sealants/rubbers with different
catalysts, viscosities, colours, setting times and set moduli.

I use a clear, one-part, non-slump paste, non-corrosive, acetoxy-cure,
high modulus RTV sealant adhesive equivalent to Dow Corning 781 RTV
which comes in normal sealant cartridges, but also available are Dow
Corning 732 in black, white and clear, and a number of others.

The high modulus types set slightly firmer than the low modulus, but
with the thin adhesive layer employed in bonding stack pneumatics this
is academic.  Set [cured] RTV is immensely tenacious yet can be parted
easily with a thin palette blade.

One-part non-slump pastes are easy and economical to use.  2-part RTV
silicones, where a catalyst is mixed in, are not so good for player work
as they are usually free-flowing and have a relatively short pot life.

GE and Würth also make equivalents and B&Q, our version of Home Depot,
sell Dow Corning products with their own label.  Beware any
silicone-based bath-type sealants that contain extenders, glossers and
and anti-fungals and which are not pure silicone RTV.

Readers will recall I stressed sealing stack and pneumatic boards first
with water-soluble PVA to achieve a better RTV bond and to completely
protect the original wood of the deck.  Even if PVA is not used, cured
RTV can easily be peeled off and the wood lightly sanded -- just as
normal -- after which hot glue will adhere perfectly.

In extremis the RTV adhesive solvent/remover available from Dow Corning
and others works well, though I have never found it necessary.

Again, please let me be clear regarding PVA that I'm am _not_ talking
about PVC-E glues like Titebond, Elmers White glue and Resin-W.  These
adhesives become fully crosslinked and water-insoluble on drying, so
cannot be removed by soaking.

For PVA I use simple Unibond PVA adhesive.  It's very consistent and
can be used straight out of the can for recovering pneumatics, or
diluted with water about 10 PVA to 1 water for sealing wood such as
stack boards to provide a septum for RTV or to make them airtight.

As well as for sealing decks and replacing shellac, PVA is the perfect
adhesive for recovering pneumatics as it has just the right grab time
yet lets the cloth relax after pulling round corners and does not make
hinges stiff, resulting in very even characteristics from pneumatic to
pneumatic.  And it is, of course, far easier and cleaner to remove next
time round than hot glue.

Anyone who has used RTV to stick on stack pneumatics -- and there are
a number now on MMD -- will vouch how easy and successful it is, how
little damage is done to the original wood and how easily it is

I cannot deprecate enough the damage done to player action stacks
through using hot glue which will be far more difficult to rectify next
time round.  And all those leaks between decks and pneumatic boards
hardly bear mentioning...

I have witnessed many enthusiasts try to remove striker pneumatics from
deck boards by cracking, splitting or chiselling them off, resulting in
cracked and split pneumatic boards and decks, which then have to be
remade or filled and sanded.  Getting enough heat through boards with
a iron to soften animal glue takes time and a knack not to char the
wood.  And I've encountered too many leaks between decks and striker
pneumatics stuck on with hot glue.

I'm sure the main reason luthiers and violin makers use hot glue is
to make a glass-hard joint between the belly parts to conduct sound
without loss; RTV and even PVA remain slightly elastic and so would
have a dampening effect.  I think you'll find that some makers today
use excellent water soluble cyanoacrylate wood adhesives for immediate
hard joints.

But I doubt that they find their instruments easier to dismantle with
hot glue -- I'm sure they soak, steam or melt sections apart, or use
benign solvents (water turps and soap) and that even then they
sometimes end up cracking thin sections.  Don't forget that neither
violins nor player stacks were actually made to come apart!

Notwithstanding the fact that certain types of silicones have been
used unadvisedly in things like breast implants, silicones are the most
amazing materials, and one of the few biocompatible.  Soon they will
be the most ubiquitous rubbers as they have the most forgiving
characteristics and are made from the second most abundant element
in the universe -- silicon.

If anyone wants details of techniques, please send me an email.

Patrick Handscombe
Wivenhoe, Essex, UK [delete ".geentroep" to reply]

 [ See the article, "Attaching Pneumatics with RTV Silicone Adhesive",
 [ at 
 [ -- Editor (Robbie)

(Message sent Tue 8 Mar 2011, 01:44:50 GMT, from time zone GMT.)

Key Words in Subject:  Adhesive, Pianola, Rebuilding, RTV, Silicone

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