Simple steps for spot-free stones

QUESTION: We made the mistake of not sealing the Arizona flagstone pavers in our backyard. Now there are black spots appearing on most of them. What do we need to do to get rid of them?

ELLYN PERREAULT

San Francisco

ANSWER: Flagstone pavers are often marred with white spots -- usually hard-water deposits -- but black spots are less common.

“My guess is that the spots are mildew or mold,” says Burt Adams, president of the Irvine-based Glaze ‘N Seal, the maker of sealers and maintenance products for concrete, tile and natural stone.

Though his company makes phosphoric acid washes that would tackle the problem, he believes they would be overkill for your situation. (They’re generally recommended for the tougher-to-lift white spots or for accidental cement smears made on the stone during installation.)

“I would use a little bit of bleach to get them off, or a citrus-based cleaner that you can buy at the supermarket,” he says. (But don’t worry, Adams promises the bleach won’t discolor your stone. Just follow his directions: Dilute the bleach, pour it over your stone, let it sit for a couple of minutes, scrub, then rinse it off.)

When the stone is cleaned and dried, seal it to prevent new stains. “You can choose a stain-resistant penetrating seal that will give your flagstone a natural look,” Adams says, “or you can opt for a film-forming sealer that will coat the surface.” With the latter, you can choose your stone’s sheen, from low gloss to a slick, wet look.

-- Christy Hobart