If your windows look streaky again within a week of washing them, the cleaner isn’t the problem — the water is. Here’s what’s actually happening on the glass.
Water spots on exterior glass come from dissolved minerals left behind after water evaporates, not from dirt. Any water source — rain, sprinklers, or a garden hose used to rinse windows — leaves some mineral residue as it dries, which is why spots often reappear within days even after a thorough hand clean.
This is one of the most common frustrations homeowners run into with DIY window cleaning: the glass looks perfect the moment you finish, and by the following week it’s hazy again in the same spots. The instinct is to blame the cleaner or the technique. Neither is usually the actual cause.
What’s actually happening on the glass
Water almost never dries completely clean. As it evaporates, whatever was dissolved in it — calcium, magnesium and other minerals — stays behind on the surface. Glass has no protective coating the way painted surfaces do, so those minerals bond directly to it. A light film wipes off easily; a film that’s been baking in the sun for a few days starts to harden, and buildup that sits for weeks can begin to etch the surface rather than just sitting on top of it.
Why regular tap water makes this worse, not better
Rinsing windows with a garden hose feels like it should help, but ordinary tap water carries the same dissolved minerals that cause spotting in the first place. Using it to "rinse away" a cleaning solution often just leaves a fresh mineral film behind once it dries — the glass looks clean for the few minutes it’s still wet, then spots reappear as it air-dries.
Why a squeegee alone doesn’t fully solve it
A careful squeegee technique removes most of the water before it has a chance to evaporate, which does help. But it rarely catches every drop, especially around window edges, corners and hardware — and any water that’s missed still dries and leaves its mineral trace. That’s why even a technically well-squeegeed window can show new spotting along the frame within a few days.
What actually prevents it: taking the minerals out of the water
The more reliable fix isn’t a different soap or a better squeegee stroke — it’s using water that doesn’t carry dissolved minerals to begin with. A triple-filtered water-fed system removes those minerals before the water ever touches the glass, so it can be left to air-dry with nothing left behind to spot as it evaporates. This is the approach we use for exterior window cleaning, specifically because it addresses the cause of spotting rather than just the symptom.
Why this matters more for exterior glass than interior
Interior windows aren’t exposed to rain, sprinklers or outdoor dust the way exterior glass is, so mineral buildup accumulates much more slowly indoors. Exterior glass gets a fresh mineral load every time it rains or a sprinkler reaches it, which is why the same spotting problem rarely shows up on the inside of the same window.
A clean exterior looks incomplete with streaky windows
Windows are one of the most visible parts of a home’s exterior, and streaked or spotted glass stands out even more once the siding around it is freshly cleaned. Many homeowners book window cleaning alongside a house wash for exactly that reason — once the siding is spotless, dull glass is the next thing anyone notices. It’s also one of the most common additions to a full exterior cleaning package.
FAQs
Will a squeegee eventually stop water spots completely?
It reduces spotting by removing most of the water before it evaporates, but it can’t remove 100% of the water every time — any missed water still dries and leaves a mineral trace, especially near edges and hardware.
Does rainwater cause the same spotting as tap water?
Yes. Rain still carries dissolved minerals and airborne particles, so it can leave the same kind of residue as tap water once it evaporates on the glass.
Is this the same thing as glass etching?
Not initially. Fresh mineral spots sit on top of the glass and can be cleaned off. Left long enough in direct sun, that buildup can start to etch into the glass surface itself, which is much harder to fully reverse — which is part of why staying ahead of it is worth doing.