Q: Two or three windows in my townhouse appear to have the screen etched onto the outside, yet I can't feel anything there. I've tried using regular glass cleaner, but nothing comes off. The windows were installed in 1998 and are welded-vinyl windows, double hung. What could this be, and how do I clean it?

Silver Spring

A: Several window-cleaning experts looked at the picture you sent and concluded that you’re dealing with corrosion that started on aluminum window screens and then got plastered onto the glass by wind and rain. Window washers call it “screen burn.”

Aluminum doesn’t rust, but when exposed to the weather, it does corrode, forming a light-colored, crusty oxide. The deposits move from the screen to the glass and build up when the windows aren’t washed for several years and the screens are left in place year-round. The glass may seem smooth, but it’s actually getting etched. “Glass is porous on a microscopic level, so stains like that can be embedded in the glass without [anyone] being able to feel them,” wrote Jacob Wallace, who staffs the help desk for ABC Window Cleaning Supply in Littleton, Colo. (800-989-4003; window-cleaning-supply.com), in response to an email asking about suitable cleaning products.

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The screen pattern in your picture is so crisp that it might seem as if a screen would have to be pressed tight to the glass, like a stencil. But forces of weather can create a distinct screen pattern even when screens are a typical distance from the glass, said Dan Diggs, owner of Window Genie, a window-cleaning company based in Bethesda (301-933-0433; bethesda.windowgenie.com).

Unfortunately, these deposits do not come off with standard window cleaner, as you have discovered. They are sometimes impossible to remove, which means the only cure then is to replace the glass. Before you resort to that, though, there are a few strategies to try. Always test a small area, maybe an inch in diameter, to make sure you aren’t scratching the glass. Wash the window first so whatever you’re scrubbing with doesn’t pick up grit, and leave water on the glass to act as a lubricant when you rub. Inspect the results of your test patch when the sun is shining on the window, because that’s when any scratches would be most visible.

When Diggs faces windows with screen burn, he begins by trying to scrape off the residue with a sharp, single-edge razor blade. When that’s not enough, he turns to an industrial diamond polish, Diamond Magic ($13.99 for a 16-ounce container at diamondmagic.com). It’s a slurry, rubbed on by hand with a soft, clean rag. Cleaning with a razor or the abrasive often goes slowly, so Window Genie charges an extra $3 on top of its typical fee of about $10 to clean a double-hung window inside and out, Diggs said. But that $3 only goes so far. If a customer wants the cleaners to keep rubbing past what they’d normally do, they keep at it — if the customer is willing to pay by the hour. “It requires a fair amount of elbow grease,” Diggs said.

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A different abrasive that some window cleaners swear by is Restoro ($9.99 for an eight-ounce container at restorowindowcleaner.com). John Koontz, whose company makes Restoro, said this is a superfine scrubbing powder — silicon dioxide screened through 325 mesh, which has openings of 0.0017 inch. It’s marketed as a cleaner for all sorts of surfaces, but Koontz said he has used it successfully on windows with screen burn. He recommends mixing the powder with water to create a paste and then rubbing it onto the glass with a terry-cloth washcloth. “It doesn’t magically take it off,” he said. “It requires a little effort.”

Wallace, at the window-cleaning-supply company, also endorses starting by trying to scrub off the oxidation, although he recommends using steel wool or a white scrub pad. (Superfine steel wool, labeled 0000, should not scratch window glass, but test it first. Don’t use coarser steel wool or a green pad used to clean pots.)

When scrubbing isn’t enough, Wallace recommends using a stain remover that consists of a mild acid and a very fine abrasive, such as his company’s Water Stain Remover ($15.84 for a 32-ounce bottle). And if that still isn’t enough, he recommends hiring a professional to use a more powerful acid, such as Crystal Clear 550. This cleaner requires careful attention to safety issues, which is why Wallace advises against homeowner use.

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Debra Martin, president of Winsol Laboratories in Seattle (800-782-5501; winsol.com), which makes Crystal Clear 550, said she usually directs homeowners with screen burn to a company that specializes in window-cleaning supplies, such as Windows101 in Seattle. The International Window Cleaning Association lists companies nationwide on its website, iwca.org. Look for the “purchasing equipment” page within its “resources” section.

These companies typically have a variety of rubbing compounds as well as acidic cleaners of various strengths. One advantage of shopping in person: If you can figure out how to remove one of the problem windows, you can take it with you and ask a staff member to guide you to suitable products. If you’re lucky, you might even be able to try out a dab or two of a couple of products to find what works. But call first and ask when the shop is least likely to be busy.

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