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Repair a Water-Damaged Finish


Repair a Water-Damaged Finish

Work miracles on wood with oxalic acid.

By Kevin Southwick

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Watering a potted plant can be disastrous if the plant lives on top of something made out of wood. We’ve all seen the white spots and black rings that can result when water seeps through the pot. And if you’ve ever tried to sand out these marks, you know it’s a tough job that can leave telltale depressions on the surface. Fortunately, in many cases, this type of damage can be almost magically undone by treating the wood’s surface with oxalic acid.

Oxalic acid removes the gray color from oxidized wood, without changing the wood’s natural color. That’s why it’s commonly used as the active ingredient in deck cleaners, and why restorers use it to remove gray or black water stains on furniture (see “Oxalic Acid Undoes Rust,” below, right). Oxalic acid is also used in some household cleaning products for removing hard water stains, and it has many industrial uses as well. Although it is found as a natural ingredient in some vegetables (spinach and rhubarb), oxalic acid is quite toxic if ingested in concentrated form.

Oxalic Acid Undoes Rust

Oxalic acid works a specific type of chemical magic by removing dark water stains from wood. These stains result when water containing iron and other minerals gets into wood. The discoloration that occurs is similar to rust. As shown here, oxalic acid is strong enough to dissolve the rust from an old plane iron, but it has little or no effect on the non-oxidized steel. (This plane iron soaked overnight in a saturated solution of oxalic acid.)

Oxalic acid is uniquely different from the other two bleaches occasionally used in wood finishing and refinishing, because neither chlorine bleach nor two-part bleach will significantly affect water stains or rust. Chlorine bleach is good for removing or lightening dyes (as it does in the laundry). Two-part bleach will lighten the color of just about anything it can soak into. Part A is sodium hydroxide (lye); Part B is hydrogen peroxide (which is also used to lighten hair color).

This story originally appeared in American Woodworker October/November 2009, issue #144.

October/November 2009, issue #144

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