Wood – Cleaning black stains from redwood

stain-removalwood

I am using redwood 4x4s for the posts on an exterior deck. EVERY 4×4 at the Home Depot is stained with black from the binding straps, etc. See pic. Obviously this is not going to work if we're going to use a transparent stain and want it to look good.

Is there an effective way to remove this type of staining from redwood without damaging the natural color of the wood?

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Best Answer

Both finewoodworking.com and landscapetutorials.com recommend using oxalic acid. The first one has a powder/paste approach, likely appropriate for individual pieces.

Start by sprinkling the oxalic acid powder over the stained area.

Then use the toothbrush dipped in water to turn the powder into a paste. Scrub the paste gently over the stained areas. Let the paste sit on the wood for 30 minutes or so, until most of the water has evaporated and the paste has turned into a white crust. You should notice the black stain has significantly lightened through the paste.

Rewet the powder with the toothbrush, then wipe off as much as you can with a clean rag. Repeat this process, wetting the paste and then wiping it off, several times to remove as much powder from the surface as you can.

The second one is more of a spray/scrub approach, more appropriate for a finished deck or fence.

Make a oxalic acid spray solution per the directions on the box.

Wet down the [wood].

[S]pray the oxalic acid solution onto the wet wood.

[R]ub it into the wood using a broom or brush. A standard plastic broom works very well. Do not use a wire brush. The black discoloration should start to fade as you rub the acid over it with the broom. Stains that are more persistent may need a second application, and a little scrubbing.

About 30 minutes after the initial acid application, rinse off the [wood] with a strong stream of water from a hose. Allow to dry and the black coloration should be gone. The entire wood surface will also be slightly lighter and brighter in color than it was previously.

As the second site notes, this could result in the wood ending up lighter and brighter than it started. If this is a concern, you'll want to treat the entire piece as uniformly as you can. Start by testing in an inconspicuous area or on a scrap piece.

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