If you’ve ever watched a professional window cleaner at work, you might have noticed something that seems a little off. After rinsing your windows with water, they pack up their equipment and leave without drying the glass.
It can seem like they’ve left the job unfinished, but there’s actually a very good reason for it.
So to clear up the confusion, let’s take a look at the science behind why window cleaners don’t typically dry the windows after cleaning, and why this method produces a far better finish than traditional drying techniques.
The key to understanding why window cleaners don’t dry the windows after cleaning isn’t actually the drying process; it’s the water itself.
Modern professional window cleaning uses a method called pure water window cleaning, which uses a special type of filtered water that has had all minerals and impurities removed.
This purified water is then delivered through a water-fed pole system, which allows the cleaner to scrub the glass with a soft brush while continuously rinsing it with pure water. Because the water contains no minerals, it can be left on the glass to dry naturally without leaving streaks or spots.
Ordinary tap water contains dissolved minerals, primarily calcium and magnesium, which are picked up as water passes through rock and soil on its way to the water supply. In many parts of the UK (especially in London and the South West), tap water is classified as “hard water,” meaning it contains a high concentration of these minerals. This is why you’ll often see limescale building up around taps and in kettles.
When you clean a window with tap water and then wipe it dry, you’re removing most of the water but leaving behind a thin film of these dissolved minerals on the glass. As the remaining moisture evaporates, those minerals are deposited on the surface, creating the white streaks and water spots that are the bane of anyone who has tried to clean their own windows. The harder you try to wipe them away, the more you smear them across the glass.
Traditional window cleaning methods using detergent and a squeegee can help reduce this problem, but they don’t eliminate it entirely, which is why there’s almost always at least some residue left behind.
Pure water (or deionised water) has had all of those dissolved minerals removed through a specialist filtration process.
What’s left behind is water in its purest form (essentially just hydrogen and oxygen molecules) with nothing else present. The result is water with a total dissolved solids reading of zero. In comparison, typical London tap water can measure anywhere between 200 and 400 parts per million.
This level of purity gives pure water a remarkable property: it’s chemically “hungry.”
Because it contains no dissolved minerals of its own, it actively seeks to absorb minerals and contaminants from whatever it comes into contact with. So when pure water touches the dirt, grime, and traffic film on your windows, it bonds with those contaminants and draws them away from the glass surface. This is why pure water is such an effective cleaning agent, even without any added detergents or chemicals. The water itself does the work naturally.
To make this easier to see, here’s a quick comparison of tap water and pure water, and why window cleaners don’t dry windows when using purified water systems.
| Why Window Cleaners Don’t Dry Windows: Tap Water vs Pure Water | ||
| Feature | Tap Water Cleaning | Pure Water Cleaning |
| Mineral content | Contains calcium and magnesium | Completely mineral-free |
| What happens when it dries | Leaves streaks and water spots | Dries clear with no residue |
| Need to wipe the glass | Usually required to reduce streaks | Not necessary |
| Cleaning chemicals | Often needed | Not required |
| Final result | Can leave smears or marks | Clear, streak-free finish |
Understanding the full cleaning process can also help explain why air-drying produces such exceptional results. The process works in three distinct stages, each building on the last to deliver a finish that traditional methods simply can’t match.
The cleaning begins with a soft brush head, fed by a continuous supply of pure water through a water-fed pole. The brush is worked across the glass in deliberate strokes, agitating the dirt and grime that has built up on the surface. This mechanical action is important because it loosens the bond between the contaminants and the glass, making it easier for the pure water to do its job. The brush also reaches into the corners and edges of window frames, where dirt tends to accumulate and where a squeegee can’t easily reach.
As the pure water flows over the glass and is agitated by the brush, it encapsulates the loosened dirt and grime. This is the chemistry at the heart of the process. The pure water molecules surround the contaminant particles, lifting them away from the glass and holding them in suspension within the water.
The dirt is effectively captured and carried away rather than simply being pushed around the surface. This encapsulation process is far more thorough than wiping with a cloth or squeegee, which tends to smear contaminants across the glass rather than removing them completely.
Once the glass and frames have been thoroughly agitated and the dirt encapsulated, a final rinse of pure water is applied to the entire window, including the glass, frames, and sills. This rinse flushes away all of the encapsulated dirt, leaving behind only pure water on a clean surface. Because the rinse water contains no minerals or impurities, there is nothing left on the glass that could cause streaks or spots as it dries. The water simply evaporates, leaving the glass in a state of perfect clarity.
This is the part that often surprises people. When pure water is left to dry naturally on the glass, it simply evaporates without leaving anything behind. And because all the minerals and impurities have been removed during filtration, there’s nothing in the water that can cause streaks, spots, or residue. As the water dries, the glass is left completely clear.
It might feel counterintuitive at first (we’re all used to wiping windows dry to prevent marks). But with pure water cleaning, touching the glass at the end of the process can actually make things worse.
For example, cloths and chamois leathers can leave behind fibres or smears, and even a perfectly clean microfibre cloth can transfer tiny amounts of residue back onto the surface. Squeegees can also leave faint lines of water along the edges of each stroke. And as those thin lines dry, they sometimes leave light marks that become visible when the sun hits the glass.
By simply rinsing the window thoroughly with pure water and allowing it to air-dry, all of those problems are avoided. There’s no final wiping stage, no cloth touching the glass, and no residue left behind. The window just dries naturally to a clear, streak-free finish.
Customers who switch to pure water window cleaning often notice the difference straight away. Their windows look genuinely clear, almost as if the glass isn’t there at all. This is especially noticeable on bright days, where there are no streaks, smears, or detergent film in sight.
Another major perk is that the results tend to last longer. Because pure water cleaning removes dirt and microscopic residues so thoroughly, there’s less left behind for new dirt to stick to. That means your windows stay cleaner for longer between visits.
These are just two of the reasons many of our customers across South West London choose regular pure water window cleaning to keep their homes looking their best.
If you’d like to see the difference pure water window cleaning can make, give us a shout!
At Stuart’s Cleaning Services, we provide reliable, professional pure water window cleaning that’s personalised to your home and always on your schedule. Get in touch today to book a one-off clean or arrange ongoing visits for your property.
As a general rule of thumb, it’s recommended to have your windows professionally cleaned every 4 to 12 weeks to keep them looking consistently clear. In areas with more traffic, pollution, or tree cover, slightly more frequent cleaning may be necessary to help prevent dirt from building up.
On a typical day, most windows dry within 10 to 20 minutes. However, warm or sunny conditions can often speed up the drying process.
Yes. Pure water cleaning is safe for all standard windows, including sash, casement, and skylights. The brushes used are soft and designed for glass and frames, and because no chemicals are involved, there’s no risk to seals, paintwork, plants, pets, or surrounding surfaces.
Streaks usually come from the minerals in tap water, mainly calcium and magnesium. When the water dries, those minerals are left behind on the glass, creating streaks and spots. In areas like London, where the water is considered hard, this is especially common. Professional window cleaners avoid this by using purified water with the minerals removed, so it dries completely clear.
Professional window cleaners often use purified (or deionised) water with all minerals removed. Because there are no impurities in the water, it can simply be left on the glass to dry naturally without leaving streaks or spots behind.
In fact, wiping the glass at the end with cloths or squeegees can sometimes make things worse by introducing smears, fibres, or residue. Letting the pure water air-dry is what allows the window to dry completely clear and streak-free.
Streaks usually come from the minerals in tap water, mainly calcium and magnesium. When the water dries, those minerals are left behind on the glass, creating streaks and spots. In areas like London, where the water is considered hard, this is especially common. Professional window cleaners avoid this by using purified water with the minerals removed, so it dries completely clear.
Yes. Pure water cleaning is safe for all standard windows, including sash, casement, and skylights. The brushes used are soft and designed for glass and frames, and because no chemicals are involved, there’s no risk to seals, paintwork, plants, pets, or surrounding surfaces.
We cover Richmond, Twickenham, Isleworth, East Sheen, Barnes, Kew, and nearby towns. Each property gets the same professional approach, clear communication, and reliable window cleaning that customers trust year after year.
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