Professional window cleaners use purified water — water with the dissolved solids stripped out of it. Tap water carries dissolved minerals that stay behind when the water evaporates, drying as the white spotting you see on a badly cleaned pane. Purified water leaves nothing behind, so the glass can be left to dry on its own with no squeegee and no detailing.
What does WFP stand for?
WFP stands for water-fed pole. It is a telescopic pole with a hose running up the inside to a brush head, fed with purified water. The cleaner scrubs and rinses the glass from the ground and walks away; the pane dries clear on its own.
What is DI, and what is a DI filter?
DI stands for de-ionised. A DI filter is a vessel packed with mixed-bed de-ionising resin that pulls the remaining dissolved ions out of water as it passes through. It is the polishing stage: reverse osmosis removes most of the dissolved solids cheaply, and the resin takes the last few parts per million.
Resin is consumed as it works, and a spent bed looks exactly like a fresh one, which is why the water is measured rather than trusted.
How pure does the water have to be?
Close to zero parts per million of total dissolved solids. That is measured with a TDS meter, a handheld probe that reads dissolved solids directly. Ordinary municipal tap water typically reads in the hundreds. Test the output before every job rather than at the start of the week.
What the meter should read
Near zero. Purified water for window cleaning should read between 0 and 2 parts per million of total dissolved solids; anything above roughly 10 ppm starts to show on the glass as it dries.
| Water | Typical TDS | Safe to leave on glass |
|---|---|---|
| Municipal tap water | 100–400 ppm | No — dries to visible spotting |
| Borehole water | often 500 ppm and up | No — worst case for spotting |
| After reverse osmosis | 5–20 ppm | Usually, on domestic glass |
| After RO plus de-ionising resin | 0–2 ppm | Yes — the professional standard |
Those numbers are why the resin stage exists at all: reverse osmosis on its own gets close, and the resin closes the last few parts per million.
Do window cleaners use distilled water?
No. Distilled water is chemically suitable but far too expensive to buy in the volumes window cleaning needs — a single commercial round can use hundreds of litres. Reverse osmosis and de-ionising resin produce water of equivalent purity on site, at a fraction of the cost per litre.
The equipment that produces it is sold as a stage in its own right: see pure water systems for reverse osmosis and de-ionising systems, and water fed poles for the poles they feed. The same purified water is what cleans solar arrays, covered under solar panel cleaning equipment.