Q (Ben): Hi Bill, I got your number from Ben Coleman at Bowens Mount Evelyn, I’ve oiled my Silver Top Ash cladding & this is what’s happened (the above hero image), have you seen it before & any ideas about what to do next? (Ben Murray, BMC)
A (Bill): Hi Ben, I have seen similar but not often after oiling. This looks like iron staining and can be quite alarming when it surfaces. It leaves black splotches or spots on the wood surface. It is actually the reaction of wood tannins coming into contact with iron in wet conditions. I only ever see this after rainy periods.
All timbers contain tannins, though some can be high tannin leecher or bleeders (think of Merbau and Blackbutt). These wood tannins are water-soluble and only released when the timber becomes wet. They can be a yellowish-brown or reddish-brown discolouration that is carried and spread by water. I easily identify it when it dries and stains surrounding concrete or tiles. When tannins come in contact with any iron or iron residue, they react to form a blue-black stain which can be seen in your photo. You can easily tell when ungalvanized nails are used in hardwood palings by the trail of black stain around the nail heads.
Another common occurrence of iron staining I have seen is on hardwood decking, resulting from the use of an angle grinder nearby which leaves iron filings on the decking surface. The iron staining only surfaces when the decking becomes wet. In severe cases, the staining can penetrate deep into the timber and become difficult to remove. But generally iron staining is mostly superficial and is easily removed with an Oxalic Acid solution. Many decking cleaning agents are Oxalic Acid based and can be used to remove the iron staining.
This also removes the need to sand or plane the affected surface. The oil coating on your Silver Top Ash cladding would have helped to protect the surface from moisture, though timber will always adjust its moisture content to its surroundings. When the moisture in the cladding is able to migrate to the surface the iron staining appears.
Replied (Ben): Thanks Bill, you know your stuff, I normally use a scour pad or sandpaper to clean off any pencil marks & stuff. But I only had steel wool on hand that day, clearly it was that! Thanks, Ben Murray.
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