Why does a window cleaner spray from the ground and leave the glass wet?

Here is the scene in the order a neighbor sees it. Water starts running at a brush on the end of a long pole. The brush moves across an upstairs pane. A last flow of water runs down the glass. Then the pole comes down and the cleaner moves on, and the pane is still wet. The water, the brush, the last rinse and the wet pane all appear in a published description, and it is not the description of a squeegee job.

The description is in NGA Glass Technical Paper FB01-00, the National Glass Association's paper on cleaning architectural glass. It records that the International Window Cleaning Association recognizes a second technique used by some professional window cleaning contractors, and it says what the technique is: “This technique employs the use of pure water delivered to the glass surface using a specialized extension pole.” The pole's job, in that sentence, is carrying water to the glass.

The water arrives through the pole rather than from a nozzle aimed up from the lawn. How the hose, the brush and the water supply fit together is set out in the parts of a water-fed pole.

The water in the hose is not meant to be straight from a spigot, either. In the technique as NGA describes it, the water running at the brush has been through ion exchange or reverse osmosis equipment on its way to the glass.

Then the brush does the part that looks like scrubbing. NGA's sentence is “Gentle agitation with a non-scratching (non-abrasive) brush is followed by the final pure water rinse.” Gentle is NGA's word, the brush it names is non-abrasive, and the order is fixed: the brush first, the final rinse after it.

The step that surprises people is the last one. NGA's description goes on: “Rinse water is generally allowed to evaporate from freshly cleaned surfaces.” The rinse is not something the cleaner forgot to take off. In the technique as NGA describes it, evaporation is generally how the rinse leaves the glass.

The plainest difference between the two methods the paper publishes shows up after the wash rather than during it. In the routine method, NGA has a window cleaning squeegee remove all of the cleaning solution from the glass immediately after it goes on. In the pure water technique, the final rinse generally stays on and dries. Walking away from wet glass is not a skipped step. In that method, it is the last step.

The same description is the basis for reasoning that a cleaner can stay on the ground. A rinse that dries by itself needs no hand at the glass to finish it; whether that makes second-story windows cleanable from the ground is answered on its own page.

Leaving water to dry only works with the right water, and NGA ties the two together: because the rinse is left to evaporate, the water is held to 20 PPM or less of total dissolved solids, and tap water is not acceptable. Why that limit matters as the rinse dries is taken up in why pure water dries without leaving spots.

From the sidewalk, water pouring down an upstairs window can look like pressure. NGA's description of the pure water technique has no pressure in it. It names a pole that delivers water, a brush that agitates gently and a rinse that is allowed to evaporate. The documents cited here publish no pressure figure for a pole rinse, and none is offered here.

The distinction matters because one group of window makers names the power washer outright. The PGT, CGI and WinDoor care and maintenance instructions put “A power washer or garden hose” first among the items not to use on their windows, screens, doors or hardware, and warn that “These improper tools may void your warranty and/or damage your products.” That is those makers' instruction for their own products. Pressure washing is a separate method, and a pole rinse as NGA describes it is treated water following a gentle brush. The garden hose on that same line has its own answer, under whether windows can be cleaned with a garden hose.

What the scene cannot tell you is what went up the hose. A pole, a brush and wet glass only match NGA's description if the water has been treated to 20 PPM or less, and that is measured, not seen. How the technique and the pole fit exterior glass is laid out under pure water and the water-fed pole on the window cleaning page.