Can windows be too dirty to clean?

Early in its cleaning paper, NGA warns that "glass products can be permanently damaged if infrequently or improperly cleaned," and says that is why glass producers and fabricators recommend strict compliance with its procedures.

Dirt on its own is not what that warning is about; it is what routine cleaning is written to handle, and NGA's trigger is condition rather than a calendar: clean glass when dirt and residue appear, and do not let dirt and residue sit on it for an extended period of time. A pane that has gone years without a cleaning carries more of that ordinary soil, and the soil is still what routine cleaning removes. What NGA's warning puts at stake after infrequent cleaning is the glass underneath, and NGA gives no length of time for it.

Where routine cleaning actually stops, and what NGA names past that point, is set out in full on how exterior glass gets cleaned. The short version: NGA's restoration section names conditions, not a level of griminess.

NGA's list of surface conditions that may call for more aggressive cleaning techniques includes, but is not limited to, an accumulation of paint, stain or varnish overspray, mortar, concrete or cement splashing, silicone sealants or lubricants smeared onto glass and frames, and sealer overspray or run-off from adjacent masonry or stone waterproofing. Those are specific materials, and the section they sit in is scoped to glass improperly stored or left unprotected during construction, so the list does not stand in for a general rule about old dirt, and it names neither salt film nor a mineral deposit as one of those conditions, though because the list is expressly not limited, their absence settles nothing either way. NGA is also plain that any non-routine cleaning carries a risk of irreparable damage to glass products, and that careful communication between the responsible parties should come before an aggressive technique is used at all. A different question this raises, whether mineral deposits fall under that same restoration heading, is answered on can mineral deposits be polished off glass, and what an exterior cleaner does and does not do once a pane crosses that line is set out on what an exterior cleaner is not.

Nothing above answers whether one particular window has crossed the line, and that is deliberate.

The PGT, CGI and WinDoor care guide tells owners to wash glass frequently to remove surface dirt and prevent stains, which points the same way as NGA's condition trigger above. Whether a given pane has gone past routine cleaning, and whether a razor blade, a chemical treatment or mechanical polishing is even appropriate to consider, is judged on the glass itself and by a glass professional, with the careful communication NGA calls for before anything aggressive is tried. Two related questions have their own pages: whether a razor blade is safe to use on glass, and why spots can survive regular window cleaning.