Why don't the spots come off with regular window cleaning?

A spot still on the glass after a wash sits in one of three places: a deposit on the outside face of the pane, a scratch in the glass itself, or a mark on the room side that exterior cleaning never reached. No source read lets a description sort those three apart.

This page sets out what is published about each and does not guess which one is on a particular window; that judgment is made on the glass by a glass professional.

USGS defines the water behind this possibility: “The simple definition of water hardness is the amount of dissolved calcium and magnesium in the water. Hard water is high in dissolved minerals, largely calcium and magnesium.” Its household example, spots or film on glasses from a dishwasher, is set out under hard water.

USGS writes those lines about dishes and glassware, not windows, and its other deposit line is about hard water heated in a home water heater. That water left to dry on a pane leaves what it carried is reasoning, not a USGS finding. What a mineral deposit is has its own entry, and the hardness of a particular home's water is the local utility's to state.

In the pure-water technique NGA records, the rinse is generally allowed to evaporate, so NGA's paper sets wash and rinse water at a total dissolved solids (TDS) content of 20 parts per million or less to prevent spotting and streaking, and rules out tap water for that technique. That number belongs to that technique only, and the pure-water technique on the window cleaning page quotes it in full.

NGA's paper makes the second possibility a matter of light, not water: scratches “that are not easily seen with a dark or gray sky may be very noticeable when the sun is at a certain angle in the sky or when the sun is low in the sky.” A spot that only appears at certain hours can be a scratch a wash cannot touch rather than a deposit a wash failed to reach; why windows look dirty when the sun is low works through that lighting effect on its own.

NGA's paper has a section headed “Non-Routine Post-Construction Cleaning & Restoration,” and the glass it describes there as beyond routine cleaning procedures is glass improperly stored or left unprotected during construction, for which razor blades, chemical cleaning or mechanical polishing may become necessary. NGA does not name hard water, salt or mineral deposits as such a condition.

Pilkington's bulletin ATS-166, written only for its Pilkington Activ self-cleaning coated glass, says the water used to rinse that glass “should not be ‘hard’,” and describes dried water spots or deposits that can appear when it dries after hosing with water that was too hard. That line stays with that one product. Whether mineral deposits can be polished off glass is a separate question with its own page.

None of the three possibilities is sorted from a description. A deposit, a scratch and a mark on the far face can all sound, in a homeowner's account, like a spot that will not come off, and no source read sorts them without a look at the pane. Irrigation water drying on glass is its own question, covered under why sprinklers spot windows.

Nothing here is a promise about a particular pane. Whether windows can be too dirty to clean takes up glass that has gone past a routine wash.