American Woodworker

Important Information >>

Syndication

Bleaching Wood

RATE THIS:

Bleaching Wood

Subtract color to add life.


Woodworkers commonly use three types of bleach: chlorine, two-part wood bleach and oxalic acid. Two-part bleach is the only one that actually changes the color of wood; the others remove stains. Read on to find out what each one does and how to use them safely.

 

Chlorine bleach

Common household laundry bleach (sodium hypochlorite) will kill mildew on your deck and outdoor furniture, and will remove dye-based stain from wood, but not pigment-based stain. Chlorine bleach can irritate skin and mucous membranes, so wear gloves and goggles.

Deck cleaner. To remove mildew from your deck or exterior furniture, fi rst hose off the wood to remove any loose debris. Mix about a quart of chlorine bleach (Clorox, Purex, etc.) to each gallon of water. Use a synthetic-bristle brush and scrub the surface with the bleach mixture. Be sure to wear goggles—it’s easy to splash. Reapply the bleach if necessary in order to keep the surface wet for about 15 minutes. Th en, brush off the surface again and hose it down thoroughly with water. Keep the runoff away from plants, pets and other wildlife.

Fortunately, deck stains are formulated with pigments, so they are not affected by the bleach. Let the wood dry completely if you plan to re-stain. If you live in an area where mildew is a problem, choose a deck stain that contains a mildewcide. Most home centers and paint stores sell them. Another option is to buy mildewcide and add it yourself.

Dye remover. Chlorine bleach will remove most dyebased stains from raw wood but will not lighten the wood itself. Th is is handy to know if you finish your project with a dye and then decide you want to “erase” it and start over. Chlorine bleach will also remove old dye you might encounter during a refinishing project.

Use a synthetic-bristle brush or a clean rag to apply the bleach full strength. It should remove the color by the time it dries, but for stubborn stains, repeat the process. If you are removing the stain from an old piece of furniture you are refinishing, make sure all the finish is off the surface and lightly scuff -sand it first. Bleach will not penetrate a finish.

As chlorine bleach dries, it breaks down to salt and water. Once the water evaporates, you’ll have salt residue on the wood. Brush it off before you finish the wood.

 

Two-part (A/B) wood bleach

Wood bleach actually lightens the color of wood. It can also de-color many pigments and dyes.

A package of wood bleach contains two bottles, usually labeled “A” and “B.” One contains lye (sodium hydroxide) and the other peroxide (hydrogen peroxide). Th e bleaching action occurs when the two chemicals come together in contact with wood. Instructions for use vary from brand to brand. Some say to put part A on first, then apply B before A dries. Other suggest mixing the two just before application. Th e object is to get both chemicals and the wood in the same place at the same time. Read the directions.

Use a synthetic-bristle brush or a clean rag to apply the bleach. When the lye goes on first, it initially darkens the wood. Once the peroxide goes on it is likely to foam as it reacts with the wood and lye. Let the wood dry completely, usually overnight, then sponge off all residue with plenty of clean water.

 

Oxalic acid

Iron, in the form of nails, hardware, or even bits of steel wool, oft en leaves a blackish stain on woods high in tannin, such as oak. A wash of oxalic solution removes these stains as well as the grayed color of oxidized wood.

Oxalic acid is sold in most hardware stores and home centers as a dry, white crystalline powder. Th e crystals are toxic and irritating to mucous membranes, so wear goggles and a dust mask when handling the dry powder. In a glass or plastic container, dissolve an ounce of oxalic acid into a pint of warm water.

Make certain that you have removed all the off ending metal before you bleach the wood. Sometimes stains are caused by broken-off nails or bits of fencing that are hidden in the wood. Wet the surface with the oxalic acid mixture and let it dry. Repeat if the stain is not completely gone. Once dry, sponge the wood with plenty of clean water to remove the crystalline residue. Any oxalic acid residue left in the wood will make irritating dust when you sand, so wear a dust mask and eye protection.

Click any image to view a larger version.

This wardrobe is made from riftsawn red oak. Several applications of two-part wood bleach turned it bone-white without obscuring the grain.


Chlorine bleach, full strength, easily removes most dye-based stain (top) but will not bleach raw wood white (center), nor will it remove pigment-based stain (bottom).



Two-part wood bleach takes the color out of most dark woods and blends the color of maple’s heartwood with its sapwood.


Oxalic acid dissolved in water removes black iron stains like magic from tannin-rich wood, such as oak.


Apply A/B bleach safely. Wear long neoprene gloves, with ends cuffed to catch drips, a waterproof apron, and goggles. Brush carefully. A/B bleach is extremely caustic and will quickly burn your skin and eyes.




This story originally appeared in American Woodworker October/November 2013, issue #168.