Often, dark stains on wood come from moisture reacting with tannin, which got its name because oak bark and oak wood, which are high in the substance, were used to tan leather for thousands of years. Tannin is also found in many fruits, vegetables and other plant materials. It is an antioxidant, and plenty of current research focuses on the health effects of eating tannin-rich foods.
Tannin is water-soluble. When wood gets soaked and then the moisture evaporates, it carries tannin to the surface, where a concentration of it is left behind. This happens most frequently with tannin-rich woods, such as oak, walnut, cherry and mahogany. Maple has relatively little tannin, but perhaps tannin in the gourd juice combined with tannin in the maple to create the stain.
Dark stains on wood can also be from mildew, which forms when wood is wet and there is a food source for the kinds of fungi we call mold or mildew. Gourd juice — like virtually all organic material — certainly qualifies as a food source.
Oxalic acid removes tannin stains, and chlorine bleach removes mildew stains. Oxalic acid is an ingredient in Bar Keepers Friend cleanser ($2.99 at Ace Hardware), but it makes up less than 10 percent of what’s in the can, according to the manufacturer’s safety data sheet. Oxalic acid is also in Bar Keepers Friend soft cleanser, but at an even lower concentration. For the undiluted form, look in paint aisles for a product such as Savogran wood bleach ($12.99 for a 12-ounce tub at Ace).
To work, though, oxalic acid and bleach need to contact the wood fibers. So furniture refinishers strip the finish first, either with solvents or by sanding. That said, it’s also clear that the stain somehow got through the finish, so you might fast-forward to the tips below about using oxalic acid to see if enough of it will penetrate to minimize the stain without stripping. One web post I found shows step-by-step pictures that indicate a black stain came off wood without stripping by using a paste of 2 parts Bar Keepers Friend cleanser to 1 part water for a few minutes, followed by a paste of half cleanser and half water. The author of the post worked in the second application with superfine 0000 steel wool, but using a synthetic pad would be safer. Steel wool can leave shards in wood pores, and tannin reacts with iron, staining adjoining wood dark.
If you are able to just treat the stain and you’re happy with the result, great! More likely, though, you won’t be able to achieve a uniform color. That’s why professionals suggest stripping the finish first, treating the stain, then refinishing.
Solvents are probably the best strippers for antiques, where preserving the patina matters. Carol Fiedler Kawaguchi, who refinishes antiques and other furniture through her Bainbridge Island, Wash., based company C-Saw, suggests using a solution of half denatured alcohol and half lacquer thinner. To protect yourself from the fumes, work outside if possible, or wear a half-face respirator fitted with organic vapor cartridges. Wear chemical-resistant gloves and eye protection. These solvents evaporate quickly, so work in sections small enough to scrape or wipe off the gooey finish before it solidifies.
Or, Kawaguchi said, you could use Citristrip safer paint and varnish stripping gel ($15.98 a quart at Home Depot). This stripper doesn’t have an obnoxious smell, it stays wet and active for hours and is labeled as safe to use indoors. Nevertheless, as the fine print on the label makes clear, ensure good ventilation and wear chemical-resistant gloves and eye protection.
If you want to avoid chemical stripping, sanding is the other option — one that might be especially appealing for a project that doesn’t involve an antique and has a flat surface without intricate molding that would make sanding difficult. Use a random-orbit sander, such as the DeWalt corded 5-inch sander with a hook-and-loop pad ($69.99 at Ace). Get a pack of medium-grit sandpaper ($11.99 for 15 Diablo-brand pads) and at least a few sheets of fine-grit sandpaper (220-grit). If possible, carry the table outside or into a garage so you don’t get sawdust everywhere. Begin with the medium-grit paper. Linseed oil reacts with oxygen in the air to create a plasticlike finish. This reaction occurs quickly at first and then slows down, continuing for years. Depending on how hardened the finish is, you might be able to sand through it easily. Or little beads of oily material might form on the sandpaper, reducing its effectiveness. Check the sandpaper frequently and change sheets as needed.
Once you’re down to bare wood, you can tackle the stain. Try oxalic acid first. The Savogran label says to mix the whole 12-ounce container into 1 gallon of hot water, but you can scale that down and mix one-fourth of the contents into 1 quart of hot water. Apply the solution to the whole tabletop, not just the stain, with a scrub brush. Wait until the wood lightens to your liking. Then rinse the surface by wiping repeatedly with a clean, damp cloth. In his book “Refinishing Furniture Made Simple,” finishing expert Jeff Jewitt says it might take several applications, with several hours of drying time between, to get the stain out.
If oxalic acid doesn’t remove the stain, try applying chlorine bleach to the stain and letting it sit overnight. If the color lightens some, but not sufficiently, repeat the process a couple of times, but maybe during the daytime so you can check periodically and end the treatment before the wood gets too bleached. Finish with a neutralizing wash of 1 part white vinegar to 2 parts water.
If the stain persists, you have three options: Call a professional refinisher; there are stronger bleaches, but they aren’t easily available. You can also sand it down until the stain is gone or at least faint enough to not bug you. Or plan on making a centerpiece a regular fixture of your table.
If you’ve used oxalic acid or bleach, the wood, once dry, will need a light, final sanding with a fine grit to remove fibers that lifted from exposure to water. If you didn’t need a sander for stripping and don’t own one, you can do this by hand, with 220-grit sandpaper. Remove all of the sanding dust and you’re ready to refinish, with linseed oil or whatever you want.
