Q: We have fall squash as a garnish on our shaped maple dining table, finished only 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 several ways to remove dark spots from wood, but you may need to try several possible solutions.
        Often, dark spots on wood are due to the reaction of moisture with tannins, which get their name from substances found in oak bark and oak wood and 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 many studies are currently 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 where the concentrated tannins remain. This happens most often with tannin-rich woods such as oak, walnut, cherry, and mahogany. Maple contains relatively few tannins, but the tannins in the pumpkin juice may have combined with the tannins in the maple to create the stain.
        Dark spots on wood can also be due to mold, which forms when the wood gets wet and is a food source for various fungi known to us as mildew or mildew. Like almost any organic material, pumpkin juice can certainly be used as a food source.
        Oxalic acid removes tannin stains and chlorine bleach removes mold stains. Oxalic acid is an ingredient in Bar Keepers Friend Cleaner ($2.99 ​​at Ace Hardware), but it makes up less than 10 percent of the can’s ingredients, according to the manufacturer’s safety data sheet. Oxalic acid is also found in Bar Keepers Friend mild detergent, but at a lower concentration. For undiluted form, look for products like Savogran Wood Bleach ($12.99 for a 12-ounce tub of Ace) in the paint aisle.
        However, the oxalic acid and bleach must come into contact with the wood fibers to work. Therefore, furniture repairers first remove the finish with solvents or sanding. However, it is clear that the stain has somehow penetrated 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 of a black stain being removed from wood without being removed with a paste of 2 parts Bar Keepers Friend cleaner and 1 part water for a few minutes, then half cleaner 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! However, it is more likely that you will not be able to get a uniform color. That’s why professionals recommend first removing the finish, treating the stain, and then refinishing.
        Thinner is probably the best way to remove antiques because the patina of antiques is important. Carol Fiedler Kawaguchi, who repairs antiques and other furniture through her Bainbridge Island, Washington-based company C-Saw, recommends using a solution of half denatured alcohol and half lacquer thinner. To protect yourself from fumes, work outdoors if possible or wear a half-mask with an organic vapor filter. Wear chemical resistant gloves and goggles. These solvents evaporate quickly, so work in small areas to scrape off or abrade the tacky finish 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 you can see from the fine print on the label, ensure good ventilation and wear chemical resistant gloves and goggles.
        If you want to avoid chemical stripping, sanding is another option – this can be especially attractive for projects that don’t involve antiques and have a flat surface without complicated sculpting that would make sanding difficult. Use a random orbital sander such as the 5″ DeWalt Corded Hook & Loop Sander ($69.99 at Ace). Buy a pack of medium grit sandpaper ($11.99 for 15 Diablo sanding wheels) 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 sandpaper. Linseed oil reacts with the oxygen in the air to form a plastic surface. This reaction proceeds quickly at first, then slows down and lasts for years. Depending on how hard the finish is, you can easily sand it down. Or, a small drop of oily substance may form on the sandpaper, which will reduce its effectiveness. Check the sandpaper frequently and replace it as needed.
        Once you have bare wood left, 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 spread the solution all over the countertop, not just the stain. Wait until the wood has lightened to your liking. Then wipe several times with a clean, damp cloth to rinse the surface. Surface preparation expert Jeff Jewitt says in his book Making Furniture Refinishing Easy that stain removal can require multiple treatments 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 enough, repeat the process several times, but perhaps throughout the day so that you can periodically check and finish the treatment before the wood becomes too discolored. Finally, neutralize and rinse 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 it down until the stain is gone, or at least light enough that it doesn’t bother you. Or plan to use the center piece as a table fixture.
        If you have used oxalic acid or bleach, after the wood has dried, a final sanding with a light, fine grit sanding will be required to remove the fibers that have floated to the surface due to water contact. 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. Remove all sanding dust, then you can touch up with linseed oil or whatever.