Colourless coatings on external timber elements, whilst very fashionable, only provide limited and short-lived protection from UV radiation in the Australian sun. As skin must be protected from UV radiation with sunscreen, timber too must be protected from UV attack by pigments in the protective coatings used.
The basics of how UV degradation occurs:
- UV radiation permanently destroys all organic substances. This includes timber even when it has been varnished with a colourless coating that has no UV protection.
- UV radiation destroys the lignin within the wood structure. Lignin constitutes about one third of the timber structure but also accounts for 80-95% of UV absorption (Nikafshar & Nejad, 2022).
- The first signs of UV degradation include changing the colour of the wood to a paler colour then finally to a silvery grey.
- When the timber has degraded to the silver-grey state the fibres within the wood structure no longer have sufficient strength to remain attached to the healthy wood underneath. It is at this point a colourless coating will simply flake off as it has no structure to hold it onto the wood substrate.
- If the degraded silvery-grey wood is simply recoated with paint without remedial sanding work, the fibres of the wood will protrude from the surface and suck in moisture. This will continue the degradation of the wood and the coating will simply flake off again. If the degraded wood is coated with another clear coating, then the silver-grey wood will change colour to a grey-black colour.
- To remedy UV degradation, a process of sanding back to healthy wood must occur and be recoated with an appropriate coating that includes sufficient pigments to mitigate UV radiation attack.
The main point is that colourless systems that contain light stabilizers are only effective for a limited time, after which UV degradation of the underlying timber will inevitably occur. Pigments are the only durable means to coat timbers and protect the wood underneath. Restoration and renovation of timbers is a costly and time intensive process that can simply be avoided by using appropriate levels of pigments in external wood coatings.
For information on the Siegware range of Adler products see Paint timber coatings.
If you have any questions or need technical advice, Andrew Ferguson (Siegware Operations Manager) is happy to help:
t: +61 3 5368 1181