Is it safe to scrape windows with a razor blade?

A pane of glass looks like a pane of glass. Three things that decide whether a blade may touch it do not show.

Whether a coating is on the outside

The National Glass Association (NGA) writes that Low-E coatings are typically neutral in color and “very difficult to see,” and that some high performance windows carry a coating on one or both exposed surfaces. Finding out where a coating sits is its own question, answered under whether Low-E windows need special cleaning.

Whether the glass is heat-treated

NGA's word for heat-strengthened or fully tempered glass is heat-treated, and Cardinal Glass Industries describes tempered glass as safety glass heated and then rapidly cooled to put its surface in compression. The mark that is supposed to identify it is not dependable: NGA writes that a logo may not be visible or present on all heat-treated glass, so a missing logo does not mean the glass is not heat-treated.

What the window's warranty says

The PGT, CGI and WinDoor care instructions name razor blades, putty knives and abrasive pads, as metal tools or sharp objects, among the items not to use on their windows, and warn that such improper tools “may void your warranty and/or damage your products.” That is those makers' sentence about their own products. Another brand's terms are in that brand's paperwork, not on the glass. How impact, Low-E, tinted and tempered panes each change the rules is set out under the glass types an exterior cleaner meets.

NGA's paper keeps metal off the glass in routine work: care is taken to avoid metal contact with the glass surface, and razor blades and metal scrapers “should not be part of routine cleaning.” Its DO NOT list adds metal parts of cleaning equipment touching the glass. As a formal position, NGA's members neither condone nor recommend scraping glass with blades or scrapers for routine cleaning, and the paper recognizes that window cleaners may choose razor blades in non-routine cleaning.

The reason behind the care is the stake NGA opens with: glass can be permanently damaged if it is cleaned improperly, which is why glass producers and fabricators ask for strict compliance with the published procedures. Scratches from routine tools and trapped grit, with no blade involved, are the subject of whether window cleaning can scratch glass.

What routine cleaning uses instead is a soak, a squeegee that removes all the solution, and a set of checks. The work runs from the top of the building down, and NGA's procedure includes checking that wind is not blowing solution onto cleaned windows. Water put on the outside of a pane runs down the glass and frame, which is why people ask whether to close windows before window cleaning. None of those steps needs metal on the glass.

For coated glass NGA writes a ban, and writes it twice, in the body of the paper and again in its DO NOT list: razor blades should never be used on coated glass surfaces. For heat-treated glass it writes a ranking instead: razor blades should be “the last possible option, especially on heat treated glass.”

Put those two rules next to the first section. A coating that is very difficult to see, and heat treatment that may have no logo to show it, are exactly the conditions under which a blade is ruled out or pushed to the end of the list. A rule that depends on what the pane is cannot be applied from what the pane looks like.

When a non-routine scrape does happen, NGA limits how: “use of razor blades should be limited to the affected areas of the glass,” in one direction only with a new blade, and never back and forth, because that motion could trap particles under the blade. The particle mechanism itself is explained under what a razor blade does to unprotected glass, and NGA notes the resulting scratches may be visible at all times or only in certain light.

NGA does describe a point past routine cleaning, and it scopes it. Glass that is improperly stored or left unprotected during construction may not come clean with routine procedures, and more aggressive techniques may become necessary, “such as the use of razor blades, chemical cleaning and/or mechanical polishing.” The conditions it names are construction soils: paint, stain or varnish overspray, mortar, concrete or cement splashing, silicone sealants or lubricants, and sealer overspray or run-off from masonry or stone waterproofing. That paragraph is written about construction, and it names no other kind of deposit.

Even inside that scope, NGA puts a conversation before the blade: “Careful communication between the responsible parties should precede the use of aggressive cleaning techniques, as any non-routine cleaning carries a risk of irreparable damage to glass products.”