Mold remover formula

How to remove black mold from concrete

A reader wants to remove black mold from this concrete wall, but is worried about nearby plants. (Photo by reader)

Question: I want to wash black mold off a concrete wall next to plants. Can I use dish detergent and a stiff bristle brush instead of a pressure washer? The water will flow to the plants, so what kind of soap should I use and how much?


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

In this case, use bleach diluted in water at the rate of three-quarters of a cup of bleach per gallon of water. Wear old clothes and put on rubber gloves. To protect plants, moisten foliage and soil before starting cleanup. This pre-wetting will dilute any solution that might settle on the leaves or on nearby soil. Then cover the plants with plastic sheeting or hold the foliage with one hand while cleaning with the other.

Sponge the bleach solution on the stained surfaces and work it with a scrub brush. Keep the stained areas damp for five minutes, blotting up more bleach solution as needed to keep it from drying out. Then rinse thoroughly with water. Also spray the leaves to dilute any drops of the cleaning solution that may have splashed in their direction.

A reader wants to remove this stain, suspected of being from a rusty can, from a bathroom sink. (Photo by reader)

Question: Before my wife and I bought our house eight years ago, we didn’t notice the stain / burn marks on the top of the bathroom sink because the owners had covered it with a box of handkerchiefs. I believe it is porcelain. Is there something we can do to remove it?


A: From the photos you sent, it looks like the stain is rust, probably from a rusty rim on a container that had a steel base. Assuming this is really the cause, a product to try is Super Iron Out Rust Remover (888-476-6688;, available as a powder and in a spray bottle. The powder formula contains citric acid as a key ingredient, while the spray is based on oxalic acid.

If you are using the powder, dampen a soft cloth and apply the powder to the cloth to make a paste. Then rub this on the stain until it is gone and rinse. With the spray, there is no friction. Just squirt, wait a few minutes, then rinse. It’s important not to let the spray dry on the surface, so don’t delay rinsing off too long. If the stain does not come off completely, just repeat the treatment.

Super Iron Out can be used safely on porcelain, glass, fiberglass and acrylic sinks. If you are unsure of the surface, test a small, inconspicuous area before applying the product to a prominent area. On a sink, the front near the overflow hole might be the best spot.

Bar Keepers Friend and Zud, both sold with other scouring powders in grocery and hardware stores, also remove rust. But they contain fine abrasives which can affect the luster of a shiny surface.