Can window cleaning scratch the glass?

NGA's clearest description of how a cleaning scratch forms is written about a razor blade: in removing tenacious contaminants from unprotected glass, particles may be trapped between the blade and the glass, resulting in fine scratches. What a blade does at the scale of a particle is laid out on the method page.

The trapped particle is not confined to blades in NGA's paper. Its quick-reference list tells window cleaners not to trap abrasive particles between the cleaning materials and the glass surface, and its routine method says the use of sufficient water will help prevent abrasive particles from being trapped between the glass and the cleaning tools being used. NGA does not treat water as the whole defense: the window cleaner, it adds, needs to be diligent in keeping all abrasive particles from scratching the glass.

Among NGA's own DO NOT items is beginning to clean without rinsing excessive dirt and debris. The PGT, CGI and WinDoor care instructions put the same idea at the start of their glass steps, for the makers' own products: begin by rinsing with clean water to remove dirt and dust particles that can cause scratches, and avoid using a power washer or garden hose.

NGA's routine method is its own step: the glass is soaked with clean water and a mild, non-abrasive solution, applied generously with a brush, strip washer or other non-abrasive applicator and lightly agitated to loosen the soil, before a window cleaning squeegee removes all of the solution. The fabric on a strip washer is the scrubber sleeve; NGA's paper does not name a scrim or any other cloth for routine glass. The published method for exterior glass sets out the full routine from the soak onward.

NGA says that during routine cleaning care should be taken to avoid metal contact with the glass surface, that razor blades and metal scrapers should not be part of routine cleaning, and, in its DO NOT list, that metal parts of cleaning equipment should not contact the glass. The same list rules out abrasive cleaning solutions or materials for maintenance cleaning.

The PGT, CGI and WinDoor instructions add a limit on the hand itself, for the makers' own products: wash with light pressure, because excessive scrubbing can lead to damage such as scratches. Whether a razor blade has any place on window glass is a separate question, answered at is it safe to use a razor blade on windows.

Where a reflective surface is exposed, NGA says special care must be taken, because damage to it may result in coating removal and a visible change in light transmittance that NGA calls very noticeable. Heat-treated glass, which NGA glosses as heat-strengthened or fully tempered, has surfaces that NGA says require greater care when cleaning.

Behind these rules sits NGA's stated reason for the whole procedure: glass can be permanently damaged if infrequently or improperly cleaned, which is why glass producers and fabricators recommend strict compliance with it. Coating removal on exposed reflective glass is taken up at can tinted windows be cleaned, and a scratch on an applied film rather than bare glass follows its own list, at can window film be damaged by cleaning.

Before a cleaning project begins, NGA strongly recommends that window cleaners test clean a small area of one window, then stop and examine the surface carefully for any damage to the glass or any exposed coating. It also says the ability to detect light scratches may vary greatly with the lighting, and that daylight conditions are needed to properly evaluate a glass surface for damage.

A scratch that is not easily seen under a dark or gray sky, NGA adds, may be very noticeable when the sun is at a certain angle or low in the sky, and such scratches may be visible at all times or only under certain lighting. Whether a particular mark on a particular pane is a scratch, a deposit or something else is for a glass professional to judge on the glass itself, in daylight, and not from a description. NGA's defenses are written to help prevent scratches, and none of them says a scratch cannot happen.