Window Washing Solution: What Works, What Doesn't, and How Professionals Get It Right

Sam White

Sep 22

9 min read

Last updated: 22 September 2026

The most effective window washing solution for professional results is purified water — specifically deionised or reverse-osmosis water — used alone or as the base for a mild detergent mix. Purified water has had the minerals stripped out, so it dries without leaving spots or streaks, which is why commercial window cleaners across the UK have moved away from tap water and soap-based mixes for exterior work.

Key takeaways

Purified (deionised or reverse-osmosis) water is the professional standard for exterior window cleaning because it dries spot-free without rinsing agents.
Hard tap water typically contains dissolved minerals that leave white residue on glass — the primary cause of streaks on freshly cleaned windows.
A pH-neutral, low-foaming detergent mixed at the correct dilution rate (typically a few millilitres per bucket) is the right choice for interior glass and heavily soiled surfaces.
Ammonia-based and solvent-heavy cleaners can damage window seals, tinted films and powder-coated frames — they are not suitable for regular commercial use.
Water-fed pole systems deliver purified water through a brush head directly to the glass, eliminating the need for squeegee work and drying time.
The wrong solution choice is one of the most common causes of streaky results — a problem Aston Maintenance regularly diagnoses when businesses call after a DIY clean goes wrong.

What actually goes into a professional window washing solution?

A professional window washing solution is not a single product — it is a combination of water quality, chemistry and dilution ratio, chosen to match the type of glass, the soiling present and the cleaning method being used.

Water quality is the foundation. Tap water in much of England — particularly across Oxfordshire, Buckinghamshire and Northamptonshire — is classified as hard or very hard, meaning it contains dissolved calcium and magnesium salts. When that water dries on glass, those minerals stay behind as white haze. Purified water, from which those minerals have been removed to near-zero parts per million, dries cleanly.

Detergent chemistry comes second. For exteriors cleaned with a water-fed pole, many professionals use purified water with no detergent at all — the brushing action and pure water do enough. For hand washing or heavily contaminated glass, a pH-neutral, low-foaming glass soap is added in small quantities, then rinsed off fully before drying.

Dilution discipline matters more than brand choice. Overdoing detergent — even a quality one — creates excessive foam, leaves a residue film, and dramatically increases the risk of streaks. The principle is: the minimum effective amount of chemistry, in the highest-quality water available.

Why does water quality affect window cleaning results so much?

Hard water is the single biggest variable between a streak-free finish and a cloudy one, and it is almost always underestimated. When water evaporates from glass, it takes nothing with it — everything dissolved in that water stays on the surface. The higher the mineral content, the worse the visible residue.

Purified water behaves differently. Because it contains almost no dissolved solids, it has a strong affinity for any contaminant on the glass surface — it effectively pulls dirt into solution more aggressively than hard tap water. When it evaporates, the glass is genuinely clean rather than coated in a fresh layer of minerals.

This is why our existing guide on streaky windows — Why Are My Exterior Windows Still Streaky After Cleaning? — points to water quality as the first thing to investigate. The solution chemistry matters, but it cannot compensate for mineral-laden water.

What are the main types of window washing solution, and when should you use each one?

Solution type	Best for	Approximate cost	Limitation
Purified water (no additive)	Exterior glass via water-fed pole	Equipment cost varies; water itself is low-cost	Requires a purification unit or bulk supply
Purified water + pH-neutral glass soap	Heavily soiled exteriors; interior hand cleaning	Detergents from a few pence per litre at correct dilution	Foam management needed; rinse fully
Tap water + professional squeegee solution	Light interior glass, quick jobs	Very low	Leaves mineral residue; streaks likely on large panes
Ammonia-based glass cleaner	Spot cleaning; small interior panes	Low retail cost	Damages rubber seals, tinted films, coated glass
Vinegar solution (DIY)	Occasional light residential use	Negligible	Acidic; can etch frames and seals over time; not professional grade
Solvent-based degreasers	Construction residue, paint overspray	Moderate	Risk of frame and seal damage; requires neutralising rinse

The professional default for commercial buildings is purified water, with detergent added only when soiling justifies it. Ammonia and solvent-based products are tools of last resort, not routine solutions.

How do professional window cleaners actually use these solutions on site?

The method determines which solution works. There are two main professional approaches for commercial buildings, and each has a different chemistry requirement. (Access methods — rope access versus cherry pickers for multi-storey work — are covered in our separate guide on Rope Access or Cherry Picker for Exterior Window Cleaning and are not revisited here.)

Water-fed pole method: how does it work with purified water?

Water is purified on-site using a vehicle-mounted or portable reverse-osmosis and deionisation filtration unit, monitored with a TDS (total dissolved solids) meter to confirm it reads near zero ppm before use.
The purified water is fed through a hollow pole — which can extend beyond 20 metres — to a soft bristle brush head pressed against the glass.
The technician scrubs the glass and frame with the brush while water flows continuously, loosening dirt and flushing it away.
The glass is left wet. Because the water is purified, it dries without leaving any residue — no squeegee work, no drying cloths required.
Frames and sills are brushed at the same time, which removes grime that would otherwise run back onto the glass in the next rainfall.

This method is the standard for ground-to-mid-rise commercial work precisely because it is fast, safe (operatives remain on the ground) and produces a consistent finish.

Traditional squeegee method: how is solution applied and removed?

A bucket of warm water is prepared with a measured quantity of professional-grade window cleaning soap — typically a low-foaming, pH-neutral concentrate.
A sleeve applicator (also called a mop or washer) is soaked in the solution and applied to the glass in overlapping strokes to wet and loosen dirt.
A rubber-bladed squeegee pulls the solution off the glass in overlapping strokes, with the blade wiped clean after each pass.
Residual water at the edges is removed with a dry chamois or lint-free cloth (called "detailing").
Any solution left on frames is wiped off immediately to prevent residue build-up.

The squeegee method is still the professional standard for interior glass and for access situations where a water-fed pole cannot reach. The discipline is in the blade technique and the chamois detailing — without both, even the best solution leaves marks.

What cleaning solutions should you avoid on commercial glass?

The chemicals most likely to cause damage are also the ones most often found in general-purpose retail cleaners — which is why commercial properties should not simply reach for whatever is under the cleaning cupboard.

Ammonia-based cleaners are effective on uncoated glass but degrade the rubber gaskets and seals around window units. Over time they also attack tinted and solar-control films, causing hazing and peeling. Many modern commercial glazing systems use coated glass that these products will dull permanently.

High-alkaline or high-acid products — including undiluted white vinegar used more aggressively than a light spray — can etch aluminium and uPVC frames, and strip protective coatings from glass. They are not suitable for routine use.

Bleach has no place in window cleaning. It does not improve cleaning performance on glass, and it will discolour frames, damage seals and run onto surrounding surfaces.

Abrasive scrubbing pads or powders scratch glass permanently. Toughened and double-glazed units are particularly vulnerable because any scratch through the surface coating scatters light and cannot be polished out without specialist intervention.

How should small businesses think about window cleaning solution as part of a service contract?

For most small business owners, the practical question is not which detergent to stock — it is whether the company they hire is using the right solution for their glazing. The wrong chemistry on coated glass or sealed units can void a glazing warranty and cause visible damage that a re-clean will not fix.

When evaluating a window cleaning contractor, three questions are worth asking:

What water quality do you use for exterior cleaning? A professional answer will reference purified or deionised water and a TDS meter reading. "We use tap water and a good soap" is not a professional answer for regular commercial work.
What solution do you use on our specific glass type? Contractors who ask about your glazing specification — coated, tinted, solar-control, heritage single-glazed — before quoting are applying the right level of care.
How do you handle frames and reveals? Detergent left on frames becomes a breeding ground for mould and staining. Professional cleaning includes frame rinse-down as standard.

At Aston Maintenance, with over 25 years in property maintenance across Oxfordshire, Buckinghamshire, London, Warwickshire and Northamptonshire, the briefing conversation before a commercial clean always covers glazing type and frame material — because the right solution depends on those specifics, not on a one-size-fits-all product.

What are the limitations of purified water systems?

Purified water is not a universal fix. In cold weather, the water-fed pole method requires care — water left on glass in sub-zero temperatures can freeze in reveals and drainage channels. Most professional operators monitor temperatures and adjust working hours accordingly.

For heavily contaminated glass — construction dust, silicone splatter, paint overspray — purified water alone will not shift the soiling. A pre-treatment with an appropriate chemical (such as a specialist silicone remover or a dilute acid wash for mineral deposits) is needed first, followed by a full purified-water rinse. Skipping the pre-treatment and running purified water over construction contamination can move the debris rather than remove it.

Purified water systems also require maintenance. Filtration membranes and resin canisters have a finite life, and a TDS reading that creeps above acceptable levels — typically anything above 10 ppm is considered worth investigating — will produce results indistinguishable from tap water. Responsible contractors check TDS before every job.

Frequently Asked Questions

What is the best window washing solution for streak-free results? Purified water — deionised or treated through reverse osmosis — is the most reliable choice for streak-free exterior glass. It dries without leaving mineral residue because the dissolved solids that cause streaks have been removed before the water reaches the glass. For interior or heavily soiled surfaces, a pH-neutral, low-foaming glass detergent at the correct dilution is the professional supplement.

Can I use washing-up liquid as a window cleaning solution? Washing-up liquid can shift surface dirt, but it is not designed for glass cleaning. It tends to foam heavily, rinse inconsistently and leave a detergent film that attracts fresh dirt quickly. It also contains ingredients that can mark frames and leave a slight haze on glass. Professional window cleaners use purpose-formulated, low-foaming concentrates for good reason.

Why do my windows look worse after cleaning them myself? The most likely cause is tap water — specifically the minerals it leaves behind when it evaporates. The second most common cause is too much detergent, which leaves a film that traps dust. Technique matters too: wiping glass dry with a cloth rather than using a proper squeegee and a chamois for edge detail tends to redistribute rather than remove the dirt.

Is purified water window cleaning suitable for all building types? Purified water via a water-fed pole is suitable for most commercial and residential buildings up to mid-rise height and works on uPVC, timber and aluminium frames. It is not appropriate as the sole method for glass with heavy construction soiling or certain types of specialist coating that require a specific chemical pre-treatment. Your cleaning contractor should assess the glazing before starting.

How often should commercial windows be cleaned to maintain appearance? Frequency depends on location, footfall and local environment. Street-facing ground-floor retail in a town centre will soil faster than an upper-floor office in a suburban business park. Monthly cleaning is common for high-visibility retail and hospitality premises; quarterly may be sufficient for less-exposed office buildings. A regular schedule prevents soiling from bonding to the glass surface and becoming harder to shift.

Does the cleaning solution affect window frame lifespan? Yes. Ammonia, bleach and undiluted acid-based cleaners degrade rubber seals and can discolour or pit uPVC and aluminium frames over time. A pH-neutral solution and a full rinse of frames after cleaning is the standard that protects glazing systems across their intended service life.

What TDS level should purified water be at for professional window cleaning? Professional-grade purified water for window cleaning is generally considered effective at 0–10 parts per million (ppm). Many operators aim for 0–5 ppm. At higher levels — particularly above 10–15 ppm — there is enough dissolved mineral content to leave visible residue as the water dries, which defeats the purpose of using purified water in the first place.

Author: This guide draws on Aston Maintenance's 25-plus years of hands-on property maintenance experience across Oxfordshire, Buckinghamshire, London, Warwickshire and Northamptonshire, covering commercial and residential window cleaning contracts.