On 19 Oct, 20:56, RJS wrote:
On 18 Oct, 22:51, The Other Mike
wrote:











If the correct release agents have been used then bonding shouldn't
ever be a problem if the joint is correctly cleaned.

Silicones in liquid, wax or spray form, simply aren't used by those
who lay up composites professionally. *Semi rigid silicones can very
occasionally be used, these are similar to baking trays / cake / jelly
moulds used in the kitchen.

This is for the simple reason that uncured silicones are really bad
news as their presence can seriously compromise the integrity of the
entire manufacturing process not only in composite sub-assembly but
also when applying paint finishes.

You need to abrade the surface and try. *If it fails it's probably for
this reason.

What remains adhered to the composite needs examining. Ideally it
needs removing back to the original finished surface but no further.
flap wheels work quite well for this task. *Then a simple degrease is
sufficient.

Aluminium when exposed to the atmosphere almost instantly forms an
oxide surface that reduces the effectiveness of the bonding.

Abrading should therefore be done in an inert atmosphere, for all
practical purposes when working on a repair, a fine scotchbrite pad
should be used for surface preparation. *Initially in the air, then
again when below the surface of a suitable solvent degreaser, then
quickly remove the component from the degreaser and wipe with a clean
lint free cloth, then immediately place in a container of catalysed
epoxy and wipe again with a piece of clean scotchbrite pad keeping the
newly abraded surface below the epoxy at all times. *Remove and bring
the two parts to be bonded together, maintaining an epoxy covering all
over the bond surface, wiping any excess that exudes from the joint.
Then maintain the parts in alignment, keeping a precise gap between
the parts until the epoxy fully cures.

Of course you could just bang the bits together and forget the
preparation, some 'experts' do just that, and members of the public,
particularly those with more money than sense, experience the failings
of just such an approach every day.

--

Can I say 'er blimey'.

But thanks for the in depth solution.

Richard *;-)

Ditto - and it does explain why I've had Al joints fail despite
cleaning, abrading, etc. Interestingly I have not had a failure with
PU glue on Al joints, admittedly in low stress situations.

What does a 'precise gap' mean; do it take it that firm clamping is
also not recommended ? Are you suggesting that the bond layer should
have minimum thickness and not a minimal thickness ?

Rob