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Work wonders on wood with oxalic acid.
Watering a potted plant can be disastrous if the plant lives on top of something wooden. 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 smooth these marks, you know it’s hard work that can leave telltale depressions on the surface. Fortunately, in many cases, this type of damage can be almost magically repaired by treating the surface of the wood with oxalic acid.
Oxalic acid removes the gray color from oxidized wood, without altering the natural color of the wood. That’s why it’s commonly used as an active ingredient in deck cleaners and why restorers use it to remove gray or black water stains on furniture (see “Oxalic Acid Cancels Rust,” below). Oxalic acid is also used in some household cleaning products to remove hard water stains and has many industrial uses as well. Although found as a natural ingredient in some vegetables (spinach and rhubarb), oxalic acid is quite toxic when ingested in concentrated form.
Identify the spots
Each final repair job is obviously unique, so the first step is to thoroughly investigate the problem. The thoracic eyelid shown here has both whitish markings (also called redness or bloom), as well as dark gray and black discoloration. The white marks are usually in the finish; the dark discolorations of the water indicate more significant damage, because they are in the woods.
To help formulate a plan to repair this finish, I moistened the entire lid with white spirit (paint thinner). This test method is useful whenever you want to closely examine old dry finish or bare wood. The appearance of the surface moistened with thinner is similar to what it would be if shellac or a clear oil-based finish were applied. In this case, wetting the surface, the whitish spots temporarily disappear (reappear when the white spirit evaporates). This means that one or two coats of finish are all that is needed to take care of the white marks. (This is a fluke; if the white marks were not gone, additional repair steps would be required to remove them, and that’s a topic for another story.)
Now I can focus on the black rings. The mineral spirits test shows that they get darker and this makes them good candidates for oxalic acid treatment. If oxalic acid works, I won’t have to aggressively sand the entire lid. Sanding would not only remove the remaining intact finish (80%, in this case), it would also lighten the aged cherry color of the lid, so it would no longer match the rest of the case it belongs to.
Each step of this treatment should be done consistently across the entire surface, not just the damaged area. The procedure involves flooding the surface with water. Fortunately, the damage shown here is on solid wood – using this treatment on a veneered surface can be risky.
The first step is thorough cleaning, using two different cleaners. White spirit removes greasy residues, such as old wax or polish (Photo 1). Delicate dish soap and water remove any water-soluble grit (Photo 2). None of these processes will damage an intact finish.
The grit is slightly raised in the water damaged area, so a little sanding with 400 grit paper is required (Photo 3). This step should take about a minute.
Mix a saturated solution of oxalic acid by adding the crystals to a jar of warm water with a plastic spoon (Photo 4). Do not use metal containers or utensils: the acid may react with the metal. Add the crystals one tablespoon at a time and mix until they no longer dissolve in the water. I always use a saturated solution, so that I have a problem solver with maximum strength.
Wearing gloves and eye protection, use a rag or sponge to saturate the entire surface to be treated, not just the dark spots (Photo 5). Keep the surface wet for about five minutes for maximum effect. Then remove the excess and allow the surface to dry completely. You may see the effect immediately as you apply the acid, or the stains may gradually disappear as the acid dries. If a second application of acid is required, wait to apply it until the first application has completely dried, otherwise it will not work. If two or three applications don’t completely remove the stain, more acid is unlikely to help. You will have to live with the remaining discoloration or resort to sanding to remove it. However, do not sand before thoroughly rinsing the surface: breathing dust that contains oxalic acid is extremely unpleasant and potentially dangerous.
After the oxalic applications, it is essential to rinse the treated surface thoroughly with plenty of clean water, to eliminate any acid left on the wood (Photo 6). Wet the surface several times and dry it each time with a clean cloth or paper towel. Let the surface dry between each flood.
Attempting to “neutralize” the acid that remains on the surface with a mild alkali such as baking soda, borax or ammonia is a cleaning option often cited as an alternative to rinsing. However, I have never seen accurate formulas that would make this chemical balance a practical possibility. Also, I know that thorough rinsing works.
After the lid has completely dried, wear a respirator and lightly sand the entire surface with 400 grit paper to remove any grains raised by oxalic treatments. Test the surface again with mineral spirits to evaluate the results (Photo 7). The test on this lid indicates that the surface is ready for finishing (Photo 8).
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