A reader is looking to remove these green copper stains from a concrete driveway. (Reader photo)

Q: Your Jan. 6 advice about using OxiClean to remove leaf stains from concrete prompts me to ask if it would remove green copper stains from a concrete driveway. The stains were caused by runoff from the roof. I’ve tried aluminum chloride in the form of a deodorant solution, which I poured directly on a 6-inch-wide band the length of the stain. Then I covered that with cornstarch. Some of the green color absorbed into the cornstarch after 20 minutes. I repeated this once and brushed it hard with detergent afterward. There was no visible change. What do you recommend for this do-it-yourselfer?

Alexandria

A: OxiClean excels at removing organic stains, such as those from leaf residue, blood, grease, grass and food. Copper isn’t on the list.

Using deodorant and cornstarch to get rid of copper stains sounds a little dubious. But you were clever to try them, as some elements of your approach do reflect advice from the General Services Administration about how to remove copper stains from concrete at historical properties, using a poultice, or paste.

The paste consists of an inert powder, such as talc or diatomaceous earth, mixed with either aluminum chloride (found in some deodorants) or ammonium chloride, moistened with full-strength ammonia or ammonium hydroxide that has been diluted with water. When this is spread over a stain, the liquid in the paste sinks into the stain and makes the copper soluble. Then, as the paste dries, the liquid moves back up to the surface, carrying the copper with it. Once the paste is completely dry, the powder and most of the stain can be scraped off, and any remaining stain can be scrubbed off with scouring powder. To find the full advisory — and its critical safety warnings — do a Google search for “GSA 111614.”

A reader wants to know how to clean this buildup between a countertop and backsplash. (Reader photo)

Pool-supply stores sell copper stain removers that can be dumped into a pool of water — far too concentrated for your purpose. There are also a few ready-to-use copper stain removers for concrete paving, but they aren’t sold by stores catering to consumers. Sure Klean T-515 Copper Stain Remover, made by Prosoco and cited in another GSA advisory (No. 112830) as effective, uses chemistry similar to the GSA recipe for a poultice. But Prosoco sells it only in quantities far too large for a job such as yours. The smallest size is five gallons of the liquid plus 25 pounds of the powder for about $1,000, said Ted Barnakoff, who answers technical questions for Prosoco. “Ninety-nine percent of our customers are contractors,” he said. “We do not really want to sell to homeowners because they will end up in the emergency room and we will get a call from their lawyer.” Asked how a homeowner could connect with a contractor who uses the product, Barnakoff said that is problematic because companies that specialize in the removal of stains from masonry usually don’t want small jobs. “They want skyscrapers and schools,” he said.

So what to do? Barnakoff said it might work to follow the general GSA procedure but with materials that you can buy easily. So you could try a poultice of ammonia and talc or diatomaceous earth (sold in garden stores as a nonpoisonous pest-control product). Protect your eyes and skin, and mix it outside in a plastic container with a wooden stick. Spread a layer an eighth-inch to a quarter-inch thick. Cover with plastic and seal the edges as best you can to slow down drying. Wait until the powder is completely dry to scrape off the poultice. This might take 24 hours, not just 20 minutes. If you remove it too soon, the copper stain will still be in the concrete. Rinse thoroughly.

Even if this approach does work, rain will continue to run off your roof, and the stains are likely to reappear unless you coat the copper roofing with lacquer — probably not the easiest thing to do or to maintain. So a completely different approach might make more sense: Consider staining your concrete green. Or just live with the stains.

My disgust levels have reached critical mass. Mold or mildew has built up on the grout between my Corian countertop and my backsplash. I have tried different cleaners, but they didn’t work. I tried a little bleach on a cotton swab, but I really had to rub to make any dent in the stains — so if I go that route, I’ll be scrubbing away for weeks! How do I get this grout clean without damaging the Corian?

Laurel

Bleach is the solution, but not with a cotton swab as the applicator.

Tear strips of paper towels, tissue or toilet paper, and find an old toothbrush. Put on rubber gloves and open a window for ventilation.

Dilute bleach in water, using half a cup of bleach to two cups of water. (Even a much higher concentration of half bleach and half water won’t hurt Corian.)

With the toothbrush, saturate a strip of paper and push it against a section of the grout. Repeat until all the areas are covered. Or, if you have a huge area to treat, you can work in sections. Leave the papers in place for at least five minutes. Push off with the toothbrush, and rinse the surface by wiping several times with a damp cloth or sponge that you rinse frequently.

Have a problem in your home? Send questions to localliving@washpost.com . Put “How To” in the subject line, tell us where you live and try to include a photo.