If your house has sprinklers, you've probably seen them: white spots on the lower panes that glass cleaner and paper towels won't touch. They aren't dirt, so scrubbing harder with the same stuff won't help. Here's what will, and what to avoid.
What the spots are
Our water carries a lot of dissolved minerals, mostly calcium. When a drop dries on glass, the water leaves and the minerals stay. A sprinkler head that hits the same pane a few mornings a week stacks those rings up until you can feel a crust with your fingernail.
It's the same scale that builds up in a kettle. That tells you two things: a mild acid dissolves it, and glass cleaner, which is made for grease and dust, does nothing to it.
Check what you have before you scrub
Clean a small patch, dry it, and look at it with light coming across the glass. Then run a fingernail over it.
- Rough and raised: minerals sitting on the glass. These can come off.
- Smooth, but still cloudy or ghosted white: the minerals have started to eat into the glass. That's etching, and nothing removes it.
We do the same check before any hard-water stain removal and tell you which one you have, so you don't pay to chase etching.
Fresh spots: try vinegar
Spotting from the last few weeks usually comes off with white vinegar. The key is giving it minutes on the glass, not seconds.
- Mix white vinegar and water half and half. Slightly warm works faster.
- Hold a soaked cloth flat on the spots for two or three minutes.
- Scrub with microfiber or a non-scratch sponge, re-wetting as you go.
- Rinse, then towel the glass completely dry so the rinse water doesn't leave its own spots.
Older spots may need two or three rounds. If they look the same after that, they've bonded past what vinegar can do, and more scrubbing will only tire you out.
What not to use
Most of the damage we see on spotted glass came from the fix, not the water. Scratches don't come out.
- Scouring pads and abrasive powders. They scratch the glass before they get through the minerals.
- Razor blades. On tempered glass they can drag grit across the surface and leave fine permanent scratches.
- Strong acid products. They can damage frames, seals and the stone or pavers below, and they're risky on tinted or coated glass.
How we take off bonded spots
On plain glass we use ultra-fine (0000) steel wool, worked wet by hand on glass we've already rinsed clean of grit. It's harder than the minerals and softer than the glass, so the spots come off without marking it. Then the pane is rinsed with deionized water, which has nothing in it to leave a new spot.
Tinted, coated and Low-E glass never gets steel wool. We use a gentler method on those panes, which is one more reason we look at every window first.
Keeping them from coming back
Find the source. Stand outside while your sprinklers run and watch where the spray lands. Usually one head has turned a few degrees, and turning it back takes five minutes. Spots on upper windows tend to come from water running down the wall above them instead.
Then don't give new spots time to bond. Our exterior window cleaning comes every two months and finishes with a pure-water rinse, so minerals come off while they still rinse off.
