A reader wants to wash black mold from this concrete wall, but is concerned about the nearby plants. (Reader photo)

Q: I want to wash black mold from a concrete wall next to plants. Can I use dish detergent and a hard bristle brush instead of a power washer? The water will be running to the plants, so what kind of soap should I use, and how much?

Potomac

A: You can certainly try washing off the mold using soapy water with about the same proportion of dishwashing soap to water as you would when washing dishes by hand. On slick surfaces, such as painted ones, this works quite well. However, if the surface is porous, as most exterior concrete surfaces are, the stains may persist.

In that case, use chlorine bleach diluted in water at a ratio of three-fourths of a cup bleach per gallon of water. Wear old clothes and put on rubber gloves. To protect the plants, dampen the foliage and soil before you begin cleaning. This pre-wetting will dilute any of the solution that might get on the leaves or on nearby soil. Then cover the plants with a sheet of plastic, or hold the foliage away with one hand while you clean with the other.

Sponge the bleach solution onto the stained surfaces and work it in with a scrub brush. Keep the stained areas damp for five minutes, sponging on more of the bleach solution as needed to keep it from drying. Then rinse thoroughly with water. Spray the leaves, too, so you dilute any drops of the cleaning solution that might have splashed in their direction.

A reader wants to remove this stain, suspected to be from a rusty canister, from a bathroom sink. (Reader photo)

Q: Before my wife and I purchased our house eight years ago, we did not notice the stain/burn marks on the top of the bathroom sink because the owners covered it with a box of tissues. I believe it is porcelain. Is there anything we can do to remove it?

Occoquan

A: From the pictures you sent, it appears that the stain is rust, probably from a rusty rim on a container that had a steel base. Assuming that’s really the cause, one product to try is Super Iron Out Rust Stain Remover (888-476-6688; www.superironout.com), available as a powder and in a spray bottle. The powder formula has citric acid as its key ingredient, while the spray is based on oxalic acid.

If you use the powder, dampen a soft cloth and apply the powder to the cloth to make a paste. Then rub that on the stain until it disappears, and rinse. With the spray, there’s no rubbing. Just squirt, wait a few minutes, then rinse. It’s important not to let the spray dry on the surface, so don’t delay too long in rinsing. If the stain doesn’t lift completely, just redo the treatment.

Super Iron Out is safe to use on porcelain, glass, fiberglass and acrylic sinks. If you’re not sure what the surface is, test a small, inconspicuous area before applying the product to a prominent place. On a sink, the front near the overflow hole might be the best place.

Bar Keepers Friend and Zud, both sold along with other scrub powders in grocery and hardware stores, also remove rust. But they contain fine abrasives that can affect the sheen of a glossy surface.