Q: We have a fall squash as a decoration on a figured maple dining table that is only oiled with linseed oil, which we apply regularly. The pumpkin leaked and left a stain. Is there a way to get rid of it?
       A: There are various ways to remove dark spots from wood, but you may need to try several possible solutions.
        Often dark stains on wood are caused by the reaction of moisture with tannins, so named because of the abundance of tannins in oak bark and oak wood, which have been used to tan leather for thousands of years. Tannins are also found in many fruits, vegetables, and other plant materials. It is an antioxidant, and much current research is focused on the health effects of eating tannin-rich foods.
        Tannins are soluble in water. As the wood soaks and the water evaporates, it brings the tannins to the surface, leaving concentrated tannins behind. This occurs most often in tannin-rich woods such as oak, walnut, cherry, and mahogany. Maple contains relatively few tannins, but perhaps the tannins in the pumpkin juice combined with the tannins in the maple create the stain.
        Dark spots on wood can also be caused by mold, which forms when the wood is damp and there is a food source for a fungus we call mold or mildew. Pumpkin juice, like almost all organic ingredients, can certainly be used as a food source.
        Oxalic acid removes tannin stains and chlorine bleach removes mold stains. Oxalic acid is in Bar Keepers Friend Cleaner ($2.99 ​​at Ace Hardware), but it makes up less than 10 percent of the package, according to the manufacturer’s safety data sheet. Oxalic acid is also present in Bar Keepers Friend mild detergent, but in a lower concentration. For undiluted form, look for products like Savogran Wood Bleach ($12.99 for a 12 ounce bath from Ace) in the paint aisle.
        However, in order to work, the oxalic acid and bleach must come into contact with the wood fibers. Therefore, furniture repairers first remove the surface coating with solvents or sanding. However, it’s clear that the stain has somehow made its way into the finish, so you can quickly jump to the oxalic acid tip below to see if enough oxalic acid has penetrated to minimize the stain without removal. A web post I found showed step by step photos showing how black spots are removed from wood without stripping, using a paste of 2 parts Bar Keepers Friend cleaner and 1 part water, stirring for a few minutes, then using half the detergent and half water. The author of this post used 0000 extra fine steel wool for the second application, but it would be safer to use a synthetic pad. The steel wool will leave splinters in the pores of the wood, and the tannins will react with the iron, turning the adjacent wood black.
        If you can handle the stain and are happy with the result, great! But, most likely, you will not be able to get an even color. This is why professionals recommend removing the finish and treating the stain before refinishing.
        For antiques, solvents are best because it is important to preserve the patina. Carol Fiedler Kawaguchi, who repairs antiques and other furniture through her company C-Saw in Bainbridge Island, Washington, recommends a solution that is half denatured alcohol and half lacquer thinner. To protect yourself from fumes, work outdoors whenever possible or wear a respirator with an organic vapor cartridge. Wear chemical resistant gloves and goggles. These solvents evaporate quickly, so work in small batches to scrape or wipe off the sticky surface before it hardens.
        Or, Kawaguchi says, you can use Citristrip Safer Paint and Varnish Striping Gel ($15.98 a liter at Home Depot). This stripper is odorless, stays wet and active for hours, and is labeled safe for indoor use. However, as the fine print on the label indicates, ensure good ventilation and wear chemical resistant gloves and goggles.
        Sanding is another option if you want to avoid chemical stripping. This can be especially attractive for projects that are not antique related and have a flat surface without intricate moldings that make sanding difficult. Use a random orbital sander, such as the DeWalt Corded 5-inch hook-and-loop pad sander ($69.99 at Ace). Buy a pack of medium grit sandpaper ($11.99 for 15 Diablo sanding discs) and at least a few sheets of fine sandpaper (220 grit). If possible, move the table outside or into the garage so the wood chips don’t get all over the place. Start with medium grain paper. Flaxseed oil reacts with the oxygen in the air, creating a plastic-like coating. This reaction proceeds quickly at first, then slows down and lasts for years. Depending on how hard the finish is, you can sand it down easily. Otherwise, small oil balls may form on the sandpaper, which will reduce its effectiveness. Check the sandpaper frequently and replace it as needed.
        Once you get to the bare tree, you can tackle the stain. Try oxalic acid first. The Savogran label says to mix the entire 12 ounce container with 1 gallon of hot water, but you can zoom out and mix a quarter of the contents with 1 liter of hot water. Use the brush to apply the solution to the entire countertop, not just the stain. Wait until the wood has faded to your liking. Then wipe several times with a clean, damp cloth, rinsing the surface. According to renovation expert Jeff Jewitt in his book Upgrading Furniture Made Easy, it may take several applications to remove a stain, with several hours of drying in between.
        If oxalic acid doesn’t remove the stain, try applying chlorine bleach to the stain and leave overnight. If the color has faded a little, but not completely, repeat the process several times, but perhaps throughout the day so that you can check and finish the treatment regularly before the wood becomes too discolored. Finally, neutralize and clean with 1 part white vinegar and 2 parts water.
        If the stain does not disappear, you have three options: call a professional painter; there are stronger bleaches, but they are not always available. You can also sand until the stain is gone, or at least light enough that it doesn’t bother you. Or plan to make the centerpiece a regular dining table fixture.
        If you have used oxalic acid or bleach, after the wood has dried, a light final sanding with fine sand will be required to remove the fibers that have floated to the surface from contact with the water. If you don’t need a sander to clean up and don’t have one, you can do it by hand with 220 grit sandpaper. Once all the sanding dust has been removed, you’re ready to touch up the surface with linseed oil or whatever.