Hard Water Stains on Florida Windows: Get Them Off Before They Etch

Search “hard water stains on windows” and you get about 880 searches a month in the US (Google Keyword Planner, October 2026). In Florida the spots are a familiar sight: a cloudy band across the lower part of the glass, or white rings that come back no matter how often you clean.

They are worth taking seriously, because the longer they sit, the harder they are to get off without hurting the glass.

What the spots are

When water dries on glass, whatever was dissolved in it stays behind. Hard water carries calcium and magnesium, and well water can also carry iron.

Florida water tends to be hard. A USGS report, now hosted by the University of South Florida, found that across much of the state the Floridan aquifer yields water with a hardness greater than 180 ppm, the level it calls “very hard.” A University of Florida guide on well water notes that iron and manganese stain fixtures at as little as 0.3 ppm. Sarasota County also uses reclaimed water for landscaping, carried in lavender or purple pipes, according to UF/IFAS Extension.

So the water that reaches your windows, from sprinklers, rain runoff off the roof or the hose, often carries plenty of minerals.

Why it matters: stains can become etching

Glass is tough, but it is not immune. Pilkington, one of the large glass makers, explains it in its cleaning bulletin: “Some deposits may not be directly harmful in themselves but by retaining moisture they can cause glass staining and etching.” It adds that other deposits do damage indirectly, by being “so firmly attached to the glass that it is damaged by the overly aggressive cleaning actions needed to remove the deposits.”

The same bulletin calls out runoff from concrete, brick and mortar, including efflorescence, as particularly harmful to glass. On a stucco house, that is worth knowing. See efflorescence on stucco.

Two takeaways: clean spots off while they are still on the surface, and do not attack old deposits with whatever is under the sink.

What the glass makers say to use

Pilkington’s routine cleaning list includes commercial glass cleaners and “a mixture of one part clear vinegar with one to ten parts clean water.” Its method:

1. Flood the glass with the cleaning solution to lift grit before you rub. Pilkington warns that a layer of abrasive dust on the glass can scratch it if too much pressure is used with too little solution.
2. Rub with a clean, lint-free cloth to dissolve the dirt.
3. Wipe dry with a clean squeegee or lint-free towel, and do not let metal squeegee holders touch the glass.
4. Rinse with distilled water if the glass still does not look clean. Pure water dries without leaving deposits of its own.

For stubborn spots, Pilkington lists specific spot cleaners, used gently and by hand, as a one-time treatment.

What never to use

• Razor blades and metal scrapers on tempered or coated glass. Pilkington: they “should not be used on heat strengthened or fully tempered glass, or coatings on glass, as fine scratches which are only visible in full sunlight can be created.” If you are not sure whether a pane is tempered, do not use a blade on it.
• Abrasive household cleaners on coated glass. Guardian’s cleaning guide for its SunGuard HD coated glass says to “NEVER use abrasive household detergents” and to keep ammonia at or under 1.5% by weight.
• Heavy scrubbing with too little water. That is how the dust on the glass becomes scratches.

If your windows have a low-E or other coating, the glass maker’s own care instructions come first.

Stop them from coming back

Cleaning only lasts as long as the water stays off the glass.

• Check the sprinklers. Run each zone and watch where it lands. A head turned a few degrees or a nozzle swapped for a shorter throw can keep the glass dry.
• Look up. Water dripping from a roof edge or an overflowing gutter leaves the same deposits.
• Rinse with clean water after you hose down the house or the lanai, so mineral-heavy water does not dry on the glass.

When it is time to call someone

If the spots are fresh, the steps above usually work. If the glass still looks cloudy after cleaning, the deposits may have started to etch it, and it is time for a professional look before anyone scrubs harder. If you have a lot of glass, tall windows or a lanai full of panels, it is a bigger job than a Saturday.

Our window cleaning covers glass inside and out, tracks and sills, and lanai and pool-cage glass, throughout Sarasota and Manatee County. Several of our Google reviews are about window cleaning. For the cage itself, see pool cage and lanai screen cleaning.