TL;DR
Streaks are residue, not dirt. They come from too much product, a cloth that is already loaded, or cleaning in direct sun so the solution dries before it can be wiped away. Use less liquid, work with two cloths -- one damp, one dry -- and clean on a cool, shaded surface.
What Actually Causes Streaks
Glass is non-porous, so nothing embeds in it. Everything you see on a pane is sitting on top: soil, or the residue of what you used to remove the soil.
Three things cause most streaking.
Too much product is the first. Cleaner that cannot be fully wiped away dries into a film, and that film is what you see in raking light. A pane needs a fine mist, not a soaking.
A saturated cloth is the second. Once a cloth is holding as much as it can, it stops removing and starts redistributing. This is why the second window looks worse than the first.
Drying too fast is the third. Sunlight on the glass evaporates the solution before you get to it, leaving everything the cleaner dissolved right where it was.
What Is the Method for Streak-Free Glass?
Clean the frame and sill first so grit does not transfer, then apply a light mist of cleaner or plain water, wipe with a damp microfiber cloth to lift the soil, and immediately buff dry with a second clean, dry cloth. Work top to bottom on a cool, shaded pane. Two cloths matter more than which product you use.
For large windows, a squeegee is faster and better: wet the glass, pull the squeegee in overlapping strokes, wiping the blade after each pass.
Product Choice and the Ammonia Question
Ammonia-based glass cleaners work well on plain glass and badly on almost everything near it.
They damage anti-reflective and anti-glare coatings, which appear on screens, eyeglasses and some specialty glass. They degrade the reflective backing of a mirror when they seep to the edges. And they must never meet a bleach-based product, since the combination releases chloramine gas.
For most household glass, a simple approach works: plain warm water with a drop of dish soap, or a light dilution of isopropyl alcohol in water for greasy marks. The alcohol evaporates quickly, which helps rather than hurts on cool glass.
Screens are a separate case entirely and get no glass cleaner at all -- covered in the electronics cleaning guide.
Mirrors: The Edge Is the Vulnerable Part
A mirror is glass with a reflective coating on the back, protected by a sealant layer. That coating is the part that fails.
Black spots creeping in from the edges, sometimes called desilvering, happen when moisture reaches the backing. That is why liquid should never be sprayed directly onto a mirror -- it runs down and wicks into the edge. Spray the cloth instead.
Bathroom mirrors face the hardest conditions: constant humidity, hairspray overspray, and toothpaste specks. Hairspray needs a solvent approach rather than more glass cleaner, usually a light alcohol solution applied to the cloth.
Fogging is a ventilation problem, not a cleaning one, and it is best addressed by running the exhaust fan, as covered in the bathroom cleaning guide.
Windows: Inside, Outside and the Tracks
Outside glass carries mineral deposits, pollen and organic film rather than the fingerprints and cooking residue found inside, so it usually needs more contact time and a proper rinse.
Hard water spotting from sprinklers is mineral and needs a mild acidic product rather than a general glass cleaner. Left for years, mineral deposits can etch glass permanently, which is why sprinkler heads aimed at windows are worth adjusting.
Window tracks are the forgotten part of the job and hold the grit that makes glass dirty again quickly. Vacuum the track first, then loosen what remains with a stiff brush and wipe.
One safety point: window cleaning above ground level is where people get hurt. Ladder work on uneven ground, or leaning to reach the far edge of a pane, is a common household injury scenario, and second-story exterior glass is generally worth handing to someone with the right equipment.
Source: U.S. Consumer Product Safety Commission data on ladder-related household injuries.
The Cluster Index: Glass and Mirror Tasks
Windows and frames
- How often should you wash windows
- How often to clean window tracks and sills
- Why do my windows look dirty again a day later
- Interior window cleaning: what it covers
- Exterior window cleaning: what it covers
- Window screen cleaning service scope
- Is window cleaning worth paying for
Mirrors and bathroom glass
- Why does my bathroom mirror spot and fog over
- How to clean a framed mirror
- How to clean dried toothpaste off a vanity
Tools and products
- What to look for in a glass cleaner
- What to look for in a window squeegee
- What to look for in a window cleaning kit
- What to look for in microfiber cloths
- Why the newspaper window trick stopped working
- Why does my microfiber cloth stop picking up dust
Decorative and household glass
- How to clean glass pendant light shades
- How to clean chandelier crystals
- How to remove candle soot from glass hurricanes
- How to clean the inside of a narrow vase
- How to clean picture frames
- Cloudy drinking glasses: film versus etching
Cloth and Tool Choice
Microfiber outperforms cotton and paper on glass for a specific reason: its split fibers grab and hold fine particles rather than pushing them around, and it leaves no lint.
Two rules keep it working. Wash microfiber without fabric softener, since softener coats the fibers and destroys their grab -- a cloth that has stopped working has usually been softened. And keep glass cloths separate from cloths used with polish or wax, because a trace of either transfers straight onto the glass as a smear.
Tool selection generally is covered in the choosing cleaning tools guide, and the diagnostic version of the streak problem is in the cleaning troubleshooting guide.
FAQ
Why does my glass streak no matter what I use?
Almost always residue. Either too much cleaner was applied, the cloth was already loaded and started redistributing, or the surface was warm enough that the solution dried before it could be wiped. Use less liquid, two cloths, and work on cool, shaded glass.
What is the best homemade glass cleaner?
Warm water with a single drop of dish soap handles most household glass, and a light dilution of isopropyl alcohol in water works well on greasy marks because it evaporates quickly. The technique matters more than the formula: light application, damp cloth, then a dry buffing cloth.
Should you spray cleaner directly on a mirror?
No. Liquid runs down the face and wicks into the edges, where it reaches the reflective backing and causes the black spotting known as desilvering. Spray the cloth instead, and wipe the mirror from the center outward rather than starting at an edge.
How do you remove hard water spots from glass?
Use a mild acidic product made for mineral deposits, applied with dwell time and then rinsed. General glass cleaner will not touch minerals. Long-standing deposits can etch the glass permanently, so adjusting sprinklers that hit windows prevents the problem recurring.
Can you use glass cleaner on a TV or a phone?
No. Ammonia and alcohol degrade the anti-glare and oleophobic coatings on modern displays, and the damage is permanent. Use a dry microfiber cloth, or one barely dampened with plain water, and never spray anything directly onto a device.
How often should windows be washed?
Twice a year suits most homes for exterior glass, with interior panes done as needed. Homes near roads, trees or sprinklers benefit from more frequent exterior cleaning, largely because mineral deposits and organic film are easier to remove before they set.
Glass rewards restraint more than effort, and most people improve their results immediately by using half as much product. When windows, mirrors and the rest of the house need handling together, our house cleaning services can cover it. Every home is different, so the real number depends on size, condition, and what you want covered -- get a quote and we will walk you through it.