Sometimes, it is necessary to bleach wood to achieve different effects with wood, or to remove unwanted stains in wood. Here, we will explain the most common bleaching treatments used today, then follow by explaining some common uses for these bleaches. Keep in mind, though, that these mixtures contain water, and will raise the wood fibers on the surface of the wood. A couple of days of drying, followed by a light sanding, are required to achieve a smooth, uniform surface prior to finishing.
Types of Commonly Used Bleaches-
Household chlorine bleach can be used for bleaching wood, but is somewhat weak in comparison to chlorine bleach products used for treatments in swimming pools (sometimes called calcium hypochlorite). Apply the bleach using a bristle brush, or wipe on with a clean cloth. Allow the bleach to penetrate for 10-15 minutes, then wipe away any excess. Repeat this process until the surface of the wood has been sufficiently bleached. Once bleaching is complete, a mixture of baking soda and water should be used to clean the surface of the wood, and stop the bleaching process.
Oxalic acid can be found at many hardware stores, and is sold as a crystalline powder (sometimes it is labeled as "Wood Bleach", check the label to see if it contains oxalic acid). Following the manufacturer's recommended mixing procedure, mix water with the oxalic acid, and apply the mixture to the wood with a brush. Allow the mixture to dry for the recommended amount of time. Sometimes this will leave a powdery residue on the wood's surface, which is okay. Repeat this process until the surface of the wood has been sufficiently bleached. Once bleaching is complete, a mixture of baking soda and water should be used the clean the surface of the wood, and stop the bleaching process.
Two Part Wood Bleach
Two part wood bleaches are commonly sold in hardware stores. They typically contain one container of hydrogen peroxide (much stronger than what is sold at most pharmacy stores), and one container of sodium hydroxide, which are mixed together to form the bleach. This bleach can be applied with a brush or clean cloth. Allow the bleach to penetrate for the recommended amount of time, then wipe away any excess. Repeat this process until the surface of the wood has been sufficiently bleached. Once bleaching is complete, a mixture of vinegar and water should be used to clean the surface of the wood, and stop the bleaching process. Please note that two part wood bleach typically does not whiten the color of wood, but rather, gives it a slight yellowish hue.
Uses for Wood Bleach -
Dye and Stain Adjustment or Removal
If wood dyes/stains are used on the wood, but the final effect is unsatisfactory, bleaches can be used to make adjustments. Full removal of dyes and stains can be difficult, and require many applications. Adjustments to the darkness of a stain, however, can be completed rather quickly.
Iron Stain Removal
Sometimes green wood can develop iron stains (which show up as blue or blackish spots in the wood) when turned. This is caused by bits of steel from a sharpened tool getting deposited on the wood surface during turning, or when tap water with some iron content is used to clean the surface of the wood when still wet. A light treatment of the entire wood surface can help reduce the visibility of these stains, or careful application with a cotton swab can treat smaller areas. Oxalic acid is particularly effective in this case. I (Mike) have personally used oxalic acid to remove iron stain rings from the surfaces of antique furniture when I used to do restoration/refinishing work.
Reducing Contrast Between Heartwood and Sapwood
In some cases, turners may need to soften the contrast between woods with differing colors of heartwood and sapwood. This can be done by applying the bleach directly to the entire heartwood, while leaving the sapwood untreated. Usually this is used in cases where the wood is to be stained after treatment, to provide the most consistent, uniform color.
Holly, maple, ash and other lightly colored woods can have nearly all of their color removed when using bleach. This is especially common when working with holly, to produce the brightest white color possible. Woods with very coarse grain such as elm, ash and white oak produce an interesting effect as well, as the well-defined grain patterns will still be visible. Skipping a final sanding after bleaching these coarse grained woods will make the grain variations stand out even more prominently.