Maintaining the quality appearance and condition of your boat

Wood Bleaching & Stain Removal

 Oxalic Acid

Oxalic acid is unique in that it will remove a certain type of stain formed when iron and moisture are exposed to tannic acid in the wood. Some woods like oak, cherry and mahogany naturally contain a high amount of tannic acid and a black stain is formed when the wood gets wet with tap water (tap water contains iron as a trace mineral). A wet glass or leaky vase left on these woods will produce a black ring. Nails and screws will form black rings around the head if the wood gets wet. If tap water is used to wet unfinished oak and mahogany, small gray spots may form on the surface of the wood. Oxalic acid will remove this discoloration without affecting the natural color of the wood.

Oxalic acid is also used to lighten the graying effects of outdoor exposure. It is the ingredient in most deck "brighteners". Used on furniture that has been stripped for re-finishing, it will lighten the color and re-establish an even tone to the wood, particularly oak.

Stain Removal

If you can determine the composition of the stain, you can remove it with the correct bleach. Iron based stains are easy to spot. They are grayish-black and usually ring shaped. It may also show up as a splotchy appearance on oak that has been stripped. Before applying the oxalic acid, remove the finish first. For normal bleaching, use a ratio of 8 ounces per gallon of hot water.  For stronger bleaching move up to 16 ounces per gallon of hot water.  Oxalic acid is the active ingredient in most single-part teak cleaners. Oxalic acid will also dull paint and fiberglass and damage anodized aluminum so wet down surrounding surfaces before you start and keep them free of the cleaner.  Apply liberally and evenly using a 3m Scotchbrite^(tm) pad or a very soft brush and scrub across the grain. Allowing the solution to remain on the surface until desired lightness has been achieved, but no longer than 30 minutes. Triple rinse thoroughly brushing the wood to clear the grain. Several applications may be needed with overnight drying in between. Once dry, it is important that any residual oxalic acid be removed from the surface of the wood before sanding or finishing. Several rinses with water will remove most of the oxalic acid crystals left on the wood surface. Neutralize the acidic wood surface with a solution made from one-quart water with two heaping tablespoons of baking soda. Stubborn stains can often be removed using Phosphoric acid.

Stains that form on wood during the drying process are varied in their composition. Sticker stain, brown stain, streaking and light "ghost" stains are all common problems and some can be removed by bleach. The composition of the stain may be chemical or microbial, so trial and error is needed when attempting to remove these stains. Start with oxalic acid, then chlorine. Last A/B bleach can be tried, but since removal or acceptable lightening of the stain results in bleaching of the surrounding wood, this is a last resort. Remember that some stains do not react to the use of bleach, so if two applications of bleach are ineffective, try a different bleach. Be sure to neutralize each bleaching by rinsing with plenty of distilled water - residual bleach can react with another - giving off nasty vapors. Stains from foods like grape juice, tea or fruits can be removed with chlorine bleach. Wipe the whole surface so that you get an even effect. Some blue and black inks based upon iron can be removed with oxalic acid, but carbon based inks such as India ink cannot be removed by any bleach.

After bleaching, you should sand with 180 or 220-grit paper to smooth out the fuzz raised by the bleaching process.

When you are not sure what the stain is, some trial and error is necessary. When faced with unknown stains follow these steps:

After stripping the old finish off and a good sanding, start with oxalic acid, then rinse it with plenty of water. Apply it to the whole surface. Wearing a dust mask, sand lightly and then see if wiping the wood with denatured alcohol leaves any stain.

If there is stain remaining, try Clorox. Make sure the oxalic acid is rinsed from the surface before applying.

If the stain is still there try the “B” part only of a two-part bleach. This is hydrogen peroxide, and if by chance the stain is from an alkali cleaner, it should lighten it.

Lastly A/B bleach can be tried, but since removal or acceptable lightening of the stain results in bleaching of the surrounding wood, this is a last resort. Remember that some stains do not react to the use of bleach, so if two applications of bleach are ineffective, move on to another type of bleach.

You can cover a lot of ground by trying oxygenated powdered bleach as your first try at an unknown problem. It is both bleach and a cleaner, and none of the products will degrade the wood. The product is readily biodegradable.