Question: We have fall pumpkin as decor on our figured maple dining table, which is covered only with linseed oil, which we apply regularly. The pumpkin leaked, leaving a stain. Is there a way to get rid of this?
Answer: There are several ways to remove dark stains from wood, but you may have to try several possible solutions.
Typically, dark stains on wood are caused by moisture reacting with tannins, so named because of the high levels 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 water soluble. When the wood is soaked and then the water evaporates, the tannins are carried to the surface, leaving behind concentrated tannins. This most often occurs in tannin-rich woods such as oak, walnut, cherry and mahogany. Maples are relatively low in tannins, but it is possible that the pumpkin sap tannins combined with the maple tannins create the stain.
Dark stains on wood can also be caused by mold, which forms when the wood is damp and there is a food source for fungus, which we call mildew or mildew. Bottle gourd sap, like almost any organic material, can certainly be used as a food source.
Oxalic acid removes tannin stains, and chlorine bleach removes mold stains. Oxalic acid is included in Bar Keepers Friend Cleaner ($2.99 at Ace Hardware), but the amount of oxalic acid in the can is less than 10 percent, according to the manufacturer’s MSDS. Bar Keepers Friend Gentle Cleanser also contains oxalic acid, but in lower concentrations. If you want the undiluted form, look for products like Savogran Wood Bleach ($12.99 for a 12-ounce can at Ace) in the paint aisle.
However, oxalic acid and bleach must come into contact with wood fibers to be effective. Therefore, furniture repair professionals first remove the surface coating using solvents or sanding. However, it is clear that the stain has somehow been absorbed into the topcoat, so you can skip to the oxalic acid tips below to see if enough oxalic acid penetrates to minimize the stain without removal. One web post I found shows step-by-step pictures of how to remove black stains from wood without stripping: Using a paste of 2 parts Bar Keepers Friend Cleaner and 1 part water, stir for a few minutes, then use half detergent and half. water. For the second application, the author of the post used ultra-fine 0000 steel wool, but it would have been safer to use a synthetic pad. Steel wool can leave splinters in the pores of the wood, and tannins can react with the iron, turning adjacent wood black.
If you can handle the stain and are happy with the result, that’s great! But, most likely, you will not get an even color. This is why professionals recommend removing the topcoat and treating the stain before painting.
For antiques, solvents are probably the best removers since protecting the patina is important. Carol Fiedler Kawaguchi, who polishes antiques and other furniture through her company C-Saw on Bainbridge Island, Wash., recommends using a solution that’s half denatured alcohol and half lacquer thinner. To protect yourself from fumes, work outdoors whenever possible or wear a half-mask respirator with an organic vapor cartridge. Wear chemical resistant gloves and goggles. These solvents evaporate quickly, so work in batches small enough to scrape or wipe off the sticky surface before it hardens.
Or, Kawaguchi says, you can use Citristrip Safer Paint and Varnish Remover Gel ($15.98 a quart at The Home Depot). This stripper has no unpleasant odor, 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.
If you want to avoid chemical removal, sanding is another option that can be especially attractive for projects that don’t involve antiques and that have flat surfaces without complex molding that makes sanding difficult. Use a random orbital sander, such as the DeWalt Corded 5-Inch Sander with Velcro Attachment ($69.99 at Ace). Buy a pack of medium-grit sandpaper ($11.99 for 15 Diablo brand sanding pads) and at least a few sheets of fine-grit sandpaper (220 grit). If possible, move the table outside or into the garage to avoid wood chips lying around. Start with medium-grain paper. Flaxseed oil reacts with oxygen in the air to create a plastic-like coating. This reaction occurs quickly at first and then slows down over several years. Depending on how hard the coating is, it can be easily sanded. Otherwise, small oil beads may form on the sandpaper, reducing its effectiveness. Check the sandpaper regularly and replace it if necessary.
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 scale it down and mix a quarter of the contents with 1 quart of hot water. Use the brush to apply the solution to the entire table, not just the stain. Wait until the wood is lighter to your liking. Then wipe with a clean, damp cloth several times to rinse the surface. Refinishing expert Jeff Jewitt states in his book Furniture Restoration Made Easy that several applications with several hours of drying time may be required to remove a stain.
If oxalic acid does not remove the stain, try applying chlorine bleach to the stain and leaving it overnight. If the color has lightened a little but not enough, repeat the process several times, but perhaps throughout the day so you can check regularly and finish the treatment before the wood becomes too discolored. Finally, neutralize and clean with 1 part white vinegar and 2 parts water.
If the stain persists, you have three options: Call a professional repairman; stronger bleach is available but difficult to find. You can also sand it until the stain is gone or at least light enough that it doesn’t bother you. Or plan to use the centerpiece as a regular fixture on the dining table.
If you used oxalic acid or bleach, once the wood is dry, it will need to be lightly sanded with fine sand to remove any fibers that have been raised by the water. If you don’t need a sander to strip and don’t have a sander, you can do it by hand using 220 grit sandpaper. Remove any sanding dust and then touch up the surface with linseed oil or something else.