TGIF: Wood Bleach Tuesday, Dec 5, 2017

It seems like many things in finishes come in threes -- three types of finishes, three types of applicators, three types of stains, three ways to color, etc.   Bleaches are no exceptions.

Basic three types of bleach, each with different uses and properties.

1.  A:B bleaches

2. Chlorine bleaches

3. Oxalic acid.

Before using any of these, be sure to protect yourself with skin and eye protection, breathing protection and air circulation.  And protect your clothing.  All require some kind of neutralization and/or rinse following application.  All these bleaches are water-based so they will raise the grain and you will need to dry and lightly sand afterwards.

You most often use bleaches when stripping and refinishing a piece, but also when certain problems occur due to storage, drying, or processing wood.  You generally want to bleach the whole piece and not just "spot bleach" or you will have problems blending in to a uniformly colored finish.

Some stains only react with certain bleaches.  And the stain may be the result of a chemical reaction or biological (mold, fungus, mouse urine, etc.)  If one or two applications is not doing it, rinse the wood with water (after the suggested neutralization) and try another.  You should use distilled water, especially if you have hard water.

A:B Bleaches

A:B Bleaches are two-part bleaches.  Part A is usually sodium hydroxide, NaOH (AKA caustic soda or lye.) Part B is hydrogen peroxide, H₂O₂. 

You apply them part A, then immediately part B, or you can mix the two together in a non-metal container and apply at once.  (Remember that lye solutions can darken certain woods, so don't wait too long).  You can also apply a second application of part B while the original solution is on and still wet.

After the wood is dry, neutralize the alkaline residue of this bleach by applying a weak acid like vinegar thinned 2:1 with water.

A:B Bleaches will overall lighten the wood and reduce some of the grain contrast.

Chlorine Bleaches

Laundry bleach (e.g., Clorox), sodium hypochlorite, NaClO, is a weaker type of chlorine bleach.   A stronger version is a pool bleach, calcium hypochlorite, Ca(ClO)₂, that you can get at pool supply stores.

To use, stir a pool bleach in hot water until you get a saturated solution (will not dissolve any more bleach powder).  Apply liberally to wood soon after mixing.   It may need to work overnight.   You can repeat once if needed. Additional applications beyond two are usually not effective. Using laundry bleach (Clorox) is usually not effective except on the lightest of stains.

Chlorine bleaches are best used for removing dye-based stains but not effective on pigment-based stains.   They are also effective on food or beverages that have penetrated a finish and made a stain.  It does not obscure the natural color variations in the wood.

Oxalic Acid

Oxalic acid's (C₂H₂O₄) main use is to remove black stains caused by iron reacting to tannins in certain woods like oak and cherry.  This is common around nails and screws or where iron pipe clamps touch wet glue when during glue-ups, or around wet glasses, etc.   It can also remove some of the graying due to weather and sun exposure.

Mix a saturated solution (see above) in hot water and apply immediately to stained area and whole piece.

When the piece is dry, neutralize the acid with a base (baking soda in water).  Then rinse.   The acid crystals remain in the wood if you don't neutralize and when sanded can be noxious to breathe.