I have brown stains on my concrete under the coated timber

These stains can be mistaken for the coating washing off in water or rain. However, the stains are tannin, leaching out of the timber. Tannins are water-soluble extractives present in some timbers such as Merbau, Kwila, Western Red Cedar, Blackbutt and Tallowwood. These tannins are leached out by water or moisture and can cause staining of surrounding areas.

To remove tannin stains from concrete, render or pavers:

• Wash affected areas with water, while scrubbing with a stiff bristle brush or broom.
• Severe staining may require multiple washes.
• For small areas, a wet cloth can be used.

For horizontal surfaces, the washed water must be removed or rinsed off quickly. This is because the tannins would be mixed with the water, hence if the water was left on the surface and dries, the tannins would simply re-deposit itself on the concrete or pavers again. Using an oxalic acid based cleaner is not recommended in these situations as they may react with the concrete, render or pavers causing another discolouration. This discolouration would be much harder to remove.