Restoration Workshop: Wood bleach
What you need to know about wood bleach
There are times when stripping a piece of furniture reveals hidden and not-so-marvelous details about construction, the work habits of the original craftsman, or the uses and abuses of past generations of repairmen and owners. These details can be totally unexpected, especially if the piece is dark in colour.
Measured solutions: The two chemicals that make up wood bleach, sodium hydroxide and hydrogen peroxide, can be combined or brushed onto the workpiece separately
What I'm thinking of are things like different species used in the same tabletop, a strong sapwood-to-heartwood contrast, or wood that was dyed before it was finished so that now the colour is locked in the wood. I've even found stains from unknown spilled substances that were just sprayed over with coloured lacquer. The stains weren't visible until the finish (including the colour coats) was stripped off.
If any of these problems exist in your project, there are at least four possible courses of action.
The simplest solution is to allow the disparate pieces, heartwood and sapwood, and unidentifiable stains, to exist as they are and tell the true history of how the piece was made. In other words, just apply a clear finish. However, it might be difficult to harmonize this piece with an existing roomful of furniture. If the piece of furniture is a valuable antique, you may want to think twice about removing stains that give the piece character. Removing an ink stain from a writing desk, for example, may actually lower its value.
The second solution, also simple, is to paint the furniture.
A third solution is to colour the lighter wood to match the darker wood, and then apply a clear finish. This is usually easier in theory than in practice. You must be careful not to add too much colour, as well as to keep the stain off the darker wood you are trying to match. I find an airbrush is the best tool to use in this instance.
The fourth solution is to lighten the dark wood to match the light wood. This may sound improbable but in fact can be done quite handily with the appropriate type of bleach. The hard part is setting up a safe environment to work in. Proper ventilation (do it outside if possible), protective clothing, rubber gloves, a respirator with cartridge filters approved for working with ammonia, and goggles (not safety glasses as you need protection from the fumes, not just splatters) are absolutely necessary.
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.
Chlorine bleach removes (or lightens) most dye stains without affecting the natural colour of the wood. It is readily available in a weak state in laundry bleach, and in the powdered form used in swimming pools. The powder intended for swimming pools is stronger and will likely require fewer applications. Mix the powder with hot water in a glass or plastic container until no more powder will dissolve'what you want is a saturated solution. Apply a liberal coat. Wait ten to twelve hours to see the full effects of the bleaching. Apply more if necessary. When the wood is the colour you want, rinse the wood several times with water. Chlorine tends to work well with dye stains but has little or no effect on pigment stains. (Pigments are large opaque particles that stay on the surface, dyes are much smaller particles that soak into the wood.)
Of course, most surface discoloration can be sanded out. But there's a limit to the amount of sanding you can do on veneer, and sanding can't change oak to maple. There may be a time when bleaching is the only available solution.
I read about a guy who was refinishing a little round table. The top was veneered in matching mirror-image quarters. One of the quarters had been stained with a bit of spilled india ink. Instead of trying to remove it, he carefully drew in matching mirror image india ink splotches on the other quarters. When he was finished, it looked like natural colour variation in the veneer. I have also watched a talented refinisher carefully change a small stain into a knot using an artist's brush and some dark brown stain. It was a pine coffee table. When he was finished, the 'knot' was totally unnoticeable.