Wood surfaces are occasionally affected by various organic and inorganic stains.....Sometimes such stains enhance the color and appearance of wood. For example, oak wood affected by the beef-steak fungus has a deep rich, attractive, brown color and there is no reason to remove the stain from the wood prior to finishing. The same applies to spalted wood whose attractive appearance is again caused by fungi. On the other hand some fungal stains and those caused by the reaction of iron with wood can disfigure wood.
These stains can be removed from wood using bleach. Bleaches are also occasionally used to reduce the difference in color between lighter sapwood and heartwood and also color variation within heartwood. Such bleaching make it easier to achieve a uniformly colored wood when the wood is subsequently colored with pigmented stains and dyes .
Furthermore, the natural colors of wood fade when wood is exposed to sunlight, and more permanent colors can be created by bleaching wood to remove its natural color and then re-coloring the wood using artificial, light-fast, stains.
The bleaches used to remove unwanted fungal stains from wood include two-part peroxide bleach and solutions of sodium hypochlorite. The former is particularly effective at removing the natural color of wood before it is recolored with pigmented stains or dyes. Oxalic acid is particularly effective at removing iron stains from wood.
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