Exterior Wood Finishing Guide Part 2

Posted by   Matt Wilkins
30 Aug 2019


Wood stains contain pigments to provide colour while accentuating the natural grain of the timber as well as providing protection against U V damage. They are micro porous and some wood stains also contain active biocides to prevent mould growth in or on the finish itself. However this is no substitute for a wood preservative, which focuses on doing this job best!


Think of a wood stain to timber as sun cream to us; if you sit out in full sun you will burn so you apply a sun cream/blocker with a certain sun factor rating for protection. The higher the rating the longer and better the protection, wood stains work to the same principles. The colour of the wood stain also has a bearing on the protection factor, the lighter the colour, the lower the protection, the darker the colour the higher the protection. Then we get to the point where the darker the stain, the more heat it absorbs causing the timber to swell, (depending on the species) doors and windows become difficult to open and close.

Colour pigments within the product filter out the harmful UV light, so generally if there are no pigments you have no protection causing the coating to lose its sheen. It becomes brittle, cracking occurs and ultimately coating failure.
Good quality pigments are expensive so a quality product is more costly, but you get what you pay for and better quality pigments hold their colour longer!

Manufactures produce wood stains in a range of colours including colourless, which will not give any long term UV protection. Most manufactures have now removed them from sale, however colourless does have it merits; for instance to treat exterior joinery before it leaves the joinery workshop. Timber is then protected from the weather for a short period of time enabling the joinery to be secured in place and then a coloured base coat applied followed by the top coat, or a coloured top coat only providing a lighter shade. Colourless top coats can also be applied to a fully finished system to add further protection without darkening the stain further.
Colourless top and base coats should not be used together as a finish on their own.


There are also different levels of wood stain build (meaning the film thickness when dry), low build, medium build and high build. The number of coats to apply (as instructed by the manufacturer) ensures that each coat lays down the correct thickness of material to ensure full system performance. The build from the coating is designed for a particular application.
Base coats are low build, lower viscosity; easily penetrating into the surface, sealing it and providing a matt finished base for the top coat .
Top coats are medium to high build, higher viscosity, providing a weather protective satin finish.
High build systems tend to be paint and varnishes, gloss finishes. You may also come across some high build wood stains although this type of product will provide greater UV protection it would be so heavily pigmented it would obliterate the grain of the timber and provide a paint type of finish.

Applying the wrong build of product to a substrate will give inadequate protection, also making a product go further (known as scrub out) will also produce a lower build than required.