For external timber furniture such as wall top seats and benches, timber leaching can be an unwelcome surprise. When tannins seep out of the timber, it causes timber leaching, or seepage.
Tannins are acidic chemicals held in solution within the liquid sap. They are water soluble, so moisture or water permeating through the timber will bring these tannins to the surface causing black or brownish stains. These stains then run off onto bench supports or similar causing unsightly stains.
The majority of tannins bleed from the ends of a timber board, which is to do with the cell structure. Imagine a piece of timber as made up from thousands of tiny straws. Moisture struggles to escape from the side walls of the straws but at the ends there are no barriers stopping the flow of tannins.
Rain is the cause for most leaching of tannins to occur. However, the tannins will only leach out of the outer couple of millimetres of the timber’s surface, so after 5/6 months leaching of tannins should stop. If the timber was then cut again, or planed down, the freshly exposed surface will release tannins all over again.
There is no obvious way to stop timber from leaching. However, there are measures to slow leaching and preventative measures.
To slow leaching, to best method is to introduce a regular oiling maintenance program. This will put a barrier on the outer timber to repel water going into the timber. This will not guarantee the non-loss of tannins because as the oil barrier wears away the timber will start leaching again. In fact, it is almost a false economy because rather than all the leaching occuring over 6 months, the process is lengthened to 5/6 years albeit at a slower rate.
If you did decide to oil the timber, it is advisable to dilute the oil with white spirit. Oil alone will not penetrate very far into fresh sawn timber as the moisture content will stop it.
The best way to prevent leaching is through careful timber species selection. Have you ever wondered why Oak is very prone to tannin leaching, yet the same problem doesn’t happen with Douglas Fir? The answer is that in general, white woods have a low tannin content and yellow, red or brown coloured woods tend to contain more tannins.
When leaching is first noticed, being water-soluble, tanning can be cleaned with just soapy water.
However, if the tannins have not been attended to quickly, then an oxalic acid base timber cleaning product may be required. These can be found by asking in a hardware store for timber tannin remover.
To use an oxalic acid tannin remover, dissolve 1kg of oxalic acid to 6lt of warm water, and scrub the solution onto to affected area using a stiff brush. Then thoroughly rinse the surface with water after treatment as oxalic acid is a toxic substance.