Thus, sodium percarbonate-based cleaners are effective in removing dirt, mildew and weathered grey residues. Once treated the wood returns to its natural original appearance. Certain wood species such as cedar and redwood contain natural resins known as tannins. These are water soluble materials which are reddish brown in color. Water can extract the resins from within the wood and deposit them on the surface, leaving brown or black discolorations.
Tannins can also react with iron present in fasteners or nails.
Resulting in blue-black stains. Neither chlorine bleaches nor oxygen bleaches are effective against tannin stains or iron stains. Oxalic acid, on the other hand, solubilizes tannins and iron stains and renders them colorless. Thus it is the material of choice for use on redwood or cedar. However, oxalic acid is not as effective against mildew.