Hard water leaves a fine white film wherever it dries on glass. On the outside of a St. Louis home, that often means spots on the lower panes, the sills, and the glass nearest the lawn. The minerals are the same calcium and magnesium that show up inside the house, but outdoors they arrive in a different way, mostly through sprinkler overspray, splashing from the ground, and rain that dries before it runs off. This guide explains how to tell what you are looking at, how to clean it, and how to keep the spots from coming back.
Why the Outside of Your Windows Gets Spotted
Water that carries dissolved minerals leaves them behind when it evaporates. Outside glass sees a lot of that. Sprinkler heads that hit the siding or the lower panes deposit a fine mist on every drying cycle, and the spots build up layer by layer. Rain works the same way when it dries in sun rather than running off cleanly. Windows that face south or west tend to dry fastest in hot weather, which is when mineral spotting is most visible.
Spring storms and early summer heat are common in the St. Louis area, and both can leave the glass looking worse than it did a week earlier. Watching the window edges closest to the yard tells you a lot about where the water is landing.
Telling Mineral Film From Etching
Most spots on exterior glass are mineral film, which wipes away with the right cleaner. Etching is different. It is a slight roughening or cloudy patch in the glass surface, usually from long contact with minerals or harsh cleaners, and it does not come off with a cloth. A simple check helps: wipe a small spot with a damp microfiber cloth. If the mark lifts, it is film. If it stays after cleaning and feels rough, the surface may be etched, and no cleaner will restore it.
A Simple Cleaning Routine
- Rinse the window with plain water first to remove dust and grit, so you do not drag it across the glass.
- Apply a mild acidic cleaner, such as diluted white vinegar, to a cloth rather than the glass directly, and let it sit briefly on the film.
- Wipe in straight strokes with a clean microfiber cloth.
- Rinse again with plain water and dry with a squeegee or lint-free cloth.
- Check the glass from an angle in good light before deciding whether a second pass is needed.
Work in the shade if you can. Glass that is warm from sun dries cleanup solution too quickly, which leaves its own streaks. Avoid abrasive pads, scouring powders, and strong household chemicals on glass, and check the instructions for any coated or tinted window film before using an acid-based cleaner. If a window has a protective coating, a gentler approach is usually better.
Preventing New Spots
- Adjust sprinkler heads so the spray lands on the lawn and beds rather than the house.
- Water in the early morning, when the sun has not yet heated the glass and water has time to soak in.
- Skip watering on windy days, since wind carries spray onto siding and windows.
- Redirect downspout runoff away from the foundation and lower panes when possible.
- Squeegee windows after heavy rain if they are still wet and spotting as they dry.
When to Test Your Water
If spotting shows up inside the home too, such as on shower glass or fixtures, the source may be the water supply rather than outdoor watering alone. A basic home test kit or a water report can confirm how hard your supply is. Your local water provider can share general information about supply in your area, though it does not advise on individual windows. Outdoor spotting alone is a cosmetic issue, and a regular cleaning routine is usually enough to keep it manageable.