Exterior Wood Finishing Guide Part 5 - Wood Varnishes & References
Varnishes
Varnishes provide a hard, transparent film coating and are
usually only available in a full clear gloss finish. Varnishes are not microporous
and are not flexible and this inflexibility can cause problems when the timber
moves. Small cracks can develop in the varnished coating allowing moisture to
penetrate the surface. Exterior varnishes now contain U V absorbers; this
extends the lifetime of the coating and helps to keep its glossy finish. It's
difficult to say how long a varnished coating will last but depending on its
directional aspect it can be 2-3 years before some form of maintenance is required.
Micro porous
When timber (treated or untreated) is exposed to the
weather, it will take up and loose moisture from the atmosphere as the
environment around it changes. This causes the timber to expand and contract.
Wood is a natural material, the cell structure, similar to our skin, can take
on and release moisture, it breathes. As a result of this moisture moving in
and out of the timber, the surface will start to split and develop cracks. The
rate of this cracking will depend on the rate at which the moisture content of
the timber changes and how long that moisture content is maintained before it
changes again.
The application of a protective wood stain will minimise
this moisture uptake and release this moisture in a controlled manor reducing
the possibility of surface cracking, swelling, warping and splitting creating
dimensional stability.
Tannins
Tannins within timbers are water soluble chemicals that can
leach out a brown coloured liquid when the timber becomes wet. Tea is a good
example of a tannin. Not all timbers present this problem but the main two
culprits currently popular within use in the UK are oak and idigbo. This brown
coloured liquid will bleed from the face of untreated timber when it becomes
wet and will stain any surface it drips on to. To prevent this from happening
do not leave oak, idgbo or other high tannin content timbers exposed to
moisture without a water repellent protective coating applied to the surface.
As the tannins are water soluble if you apply a water base product to the
surface the tannins will discolour the coating, which should not present too
much of a problem unless you are applying a light coloured (white, cream etc)
primer and or a top coat. In this case it is advisable to treat the timbers
first with a water based tannin blocker primer, Impra profilan secure or
oil/spirit based base coat or primer.
Tannin stains can be removed with an application of oxalic acid, depending
on the density of the staining, it may require a few applications.
