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It is common for decking timber and other exterior timbers such as cladding to generate black mould over time. In most circumstances, the mould is living on the surface of the timber, feeding on sap sugars and other extractives released by the wood over time.
One thing to bear in mind with mould, is that it is purely an aesthetic issue and is not related to decay of the wood, which is caused by a different group of organisms called fungi. So while mould doesn’t look very nice, it will not generally result in product failure.
Mould requires moisture and warmth to exist, and when persistent mould has been present on the wood for some time, the wood itself can become stained. Mould spores can be present on the wood but not be visible for some time. So it is important to control signs of mould early.
Some timbers, particularly lighter coloured species like vitex and pine will show mould more readily than others. Some modified timber products, particularly acetylated wood (such as Accoya) appear to be more susceptible to mould although they exhibit excellent resistance to wood destroying fungi.
The first step in controlling mould is to keep your exterior timber clean.
Wash your deck or timber weatherboards regularly – at least once a year minimum though every 6 months is better. This is particularly important prior to application of a coating as this prevents mould growth underneath the coating. Use warm water, mild detergent or timber cleaner apply with stiff brush and rinse off with a hose. Do not use high pressure water blasters, these can damage wood.
There are a number of timber cleaning products available commercially from your local hardware store that can help, particularly if mould growth is heavy. Sodium percarbonate-based oxygenating cleaners such as Rejuvenator, ‘Hit the Deck’ or oxalic acid cleaners such as Intergrain Reviva are effective low toxicity options. Be aware of timber cleaning products with excessive chemicals, they can persist in the wood and surrounding environment.
For persistent and recurring mould issues, a preventative long-acting mouldicide can be used. These can be in the form of a water-based solution often containing quaternary ammonium compounds such as benzalkonium chloride- brand names include ‘Wet and Forget’ or Preventor. The solution is normally applied to dry, clean wood and allowed to dry. The natural weathering process cleans the mould away over time and prevents further growth. Periodic re-application is required. A coating can be made over the top if desired once dry. Use these products with caution however as they can be toxic to plants and fish.