Restoration Workshop: Wood bleach

It would be nice if there was one type of bleach and it could do everything. Unfortunately, there isn’t. There are, in fact, three types of wood bleach, but they each do a different job and are not interchangeable. These bleaches are peroxide, oxalic acid and chlorine. Peroxide bleach usually comes in two parts, commonly labelled A and B. A is usually sodium hydroxide and B hydrogen peroxide; when the two are mixed, a strong oxidizing reaction takes place. This bleach is available in most paint stores. It’s used to remove the natural colour of wood when you have disparate species, or sapwood-to-heartwood contrast. To use, apply a coat of A (use a brush and give the wood a thorough coat, wet but don’t flood) followed immediately with a coat of B. You can mix the parts first and then apply it to the wood, but apply it immediately after mixing. Sometimes a second treatment is necessary. When the wood has dried, neutralize the bleach with a white vinegar wash. Then rinse with water. Be warned: this treatment does not work at all on some woods, like ebony, and to varying effect on others — if too much is put on, it can leave a greenish tinge on walnut. Practice on a scrap piece of the same species as your project. It also removes all the natural colour variations in the wood, so use it wisely.

Oxalic acid removes rust stains, and chlorine bleach takes off dye stains

Oxalic acid removes stains caused by iron, water and tannic acid, as in the combination of nails, rain and oak. This bleach is found in many deck brighteners. It comes in dry crystals which can be dissolved in warm water. Use a plastic or glass container for mixing. It does not affect the colour of the surrounding wood, so you can apply it beyond the actual stain. You may have to apply it several times. Allow the piece to dry thoroughly between applications. When the colour difference has been removed, rinse the wood several times with water. Then rinse with a solution of water and baking soda (one quart to two tablespoons). Then rinse one last time with water.