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.
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 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.