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Problems with Exterior Oak
American Oak, both red and white and European Oak too, are not
easy timbers to dry and are recognised as being prone to check,
split, honeycomb and caseharden.
However, because of these characteristics it is all the more
important that drying should be carried out carefully. It is equally
important that manufacturers of joinery for exterior applications should
be aware of the potential problems and how to deal with them.
Furthermore, American Oak has been exported to this country,
in several instances, with excessively low moisture content.
Levels as low as 6% m.c. being typical. For example, a
600mm wide table top made from American Oak at 6% m.c. can expand
to over 613mm by the time it reaches 12% m.c. In the UK,
timber for exterior use should be conditioned to a moisture content
of 12-20% which is appropriate for our climate.
Casehardening and honeycombing are defects that occur when the
timber has been dried too quickly by the supplier. You
have probably experienced these if you have had splitting problems,
panels that have cupped, or appeared satisfactory until stained
but then mysteriously show cracks.
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Casehardening
relates to the stresses which naturally develop when wood is dried from
the green or unseasoned state. In the early stages of drying,
the outer layers of the wood tend to loose moisture more quickly
than the interior. The outer layers want to shrink around
the inner layers which have not started to shrink because they
are wetter. This induces compressive stresses in the interior
core.
The surface layers are actually prevented from shrinking to the
extent they would wish to by the wetter, less shrunk, core.
Hence tension stresses develop in the surface layers. If
the development of these stresses is not controlled by the kilning
cycle, the outer layers become permanently set in a less than
fully shrunk condition.
As the drying process continues, the interior of the plank begins
to dry and shrink so the compressive stresses acting on the core
are reduced. However, further normal shrinkage of the interior
core may now be prevented by the outer layers of the plank if
permanent set has developed. This action reverses the distribution
of stresses in the timber, causing tension to develop in the
interior core and compression in the surface layers.
Timber in this condition is said to be casehardened and the stresses
which have been induced will always be there.
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Honeycombing
occurs when the tension stresses in the inner core become so great that the
stretched fibres are torn apart, forming internal splits. The thicker
the sections of timber are when dried, the more prone they are to casehardening
and honeycombing due to the slower rates of drying and increased moisture gradients
from surface layers to interior core.
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Relief Treatment
consists of raising the humidity and temperature at a critical point in
the drying cycle. The surface layers become wetter causing
expansion which then balances out the opposite stresses in the
core.
A quality supplier will monitor the development of casehardening
stresses and towards the end of the drying process apply a relief
treatment.
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Coating Recommendations
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Oak is not as resistant to weathering as oily hardwoods and, therefore, needs better protection
when used outdoors. The large open pores make the problem worse providing a route
for water-ingress. Blue-stain mould spores are likely to
be present on any piece of timber and will be activated by moisture. Another
problem with Oak is the high tannin content, if not properly sealed water
(rain) can wash the tannin out of the wood leaving
a patchy colour and staining to the surrounding areas (bricks,
patio, etc.).
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It is essential to use a coating that will give a high-build, seal the pores
effectively and so provide a barrier to water.
Rustin's
Flexterior
and Rustin's
Exterior Varnish
are both suitable for this purpose. These require 3 coats initially
for good durability and will not require re-coating for many
years. Wood shades are also available within the two ranges
and these give extra durability because the pigments act as UV
absorbers. A system comprising one or two coats of the
coloured coating (e.g. Flexterior - Dark Oak) followed by the
clear version (e.g. Flexterior - Clear) to a total of 3 coats
will be more durable than three coats of the clear product alone.
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Pre-conditioning to the correctmoisture content is very important.
If the Oak is outside the moisture content range of 12-20% there
will be excessive movement as it comes to equilibrium. The wood can
become split, and the coating film may also split exposing bare wood, providing
a route for water-ingress and rapid
deterioration of the coating.
If this occurs remedial action must be taken immediately to prevent
the wood becoming weather-stained. Clean the surface and
reapply sufficient coats of the same coating product to fill
the cracks/splits. If weather stains have occurred
they can be removed with Rustin's
Wood Bleach
.
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Many people prefer the look of oiled wood and so use Rustin's
Danish Oil
.
This can give the protection required if
a good film thickness is built-up initially and is maintained
by regular re-oiling, before bare wood is exposed. Oil finishes
are high maintenance finishes, and given the inherent problems with Oak,
do not provide the maximum protection offered
by products such as Rustin's Flexterior and Rustin's
Exterior Varnish.
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