In the morning, every window looks clean. Then in the afternoon the sun hits the glass at an angle, and the arcs, streaks and the haze settled in the corners appear. Most people respond by buying a stronger product, yet the product is rarely the cause of streaks. Far more often it is the timing, the tools and the order of work: in other words, the technique.
Where do the streaks come from?
A streak is always some kind of residue: cleaning product that was not fully removed, minerals left behind by the water, lint from the cloth, or smeared grease. The shape of the streak usually tells you which it is, and from that, what to do differently.
| What you see | Likely cause | What to do differently |
|---|---|---|
| Arcs and streaks following the direction of the wipe | Too much product, or a worn or dirty squeegee blade | Less product, an undamaged rubber blade, and wiping the edge after every stroke |
| Tiny white spots after drying | Hard water: the minerals in the water are deposited | Squeegee quickly and thoroughly; never leave water to dry on the glass |
| Lint and fine fibres | Paper towels or old rags | Squeegee, lint-free cloth for the edges |
| Greyish haze, visible against the light | Smeared grease or fingerprints | Dissolve the grease first, only then squeegee |
| Run marks at the bottom of the glass | Water dripping from the frame later | Frame and casing first, then the glass |
Timing: in the shade, not in direct sunlight
When the glass is warm, the cleaning solution dries faster than it can be squeegeed off, and the residue leaves streaks. On an overcast day, on the shaded side of the building, or in the morning and late afternoon when the sun is not shining directly on the glass, it is much easier. It is worth working round the windows following the sun: start with the side that is currently in shade.
In frost, however, it is not worth starting wet work outside: water can freeze on the glass and on the frame seals, leaving ice instead of streaks.
Equipment: squeegee, cloth, a little detergent
The squeegee
Professional window cleaning uses a squeegee, not a cloth. The rubber blade must be intact and flexible: a nicked or hardened blade leaves a thin streak in the same place on every stroke. The blade has to be wiped after every stroke; otherwise the next stroke carries the dirt back onto the glass.
The cloth
Paper towels and old rags leave lint and tend to smear dirt rather than pick it up. A cloth is needed at the edges and corners the squeegee cannot reach: use a clean, lint-free microfibre cloth for this — not the one used to wash the frame.
Water and cleaning products
Too much cleaning product is one of the main causes of streaks. A small amount of well-diluted product is enough; what matters is thorough squeegeeing. Hard water makes things more difficult, as the water itself leaves marks when it dries, which is why you need to squeegee quickly, with overlapping strokes. For high surfaces, we use a pure-water system, which washes with demineralised water that leaves no marks once dry.
The order, step by step
If the frame and sill are done after the glass, dirty water drips down and spoils the freshly cleaned surface. That is why the order matters at least as much as the equipment.
- Clearing the sill, then dry-removing dust and cobwebs from the frame and the track.
- Washing the frame, the casing and the drip rail, so that dirty water does not drip from them onto the glass later.
- Wet the glass with water and a little detergent to loosen the dirt; leave it slightly longer on greasy marks and fingerprints.
- Squeegeeing: first a strip along the frame, then even, overlapping strokes, wiping the rubber blade after each stroke.
- Edges and corners finished off with a lint-free cloth.
- Wiping the sill at the end, as it collects all the water that drips down.
- Inspection under side lighting, from several angles: what you can't see head-on shows up at an angle.
Streaks are rarely down to the product. They are almost always down to the method's: too much product, warm glass, the wrong order.
A few persistent myths
- “More product, cleaner glass.” It is the other way round: whatever you fail to squeegee off shows up as streaks once it dries.
- “Vinegar works on everything.” An acidic product can permanently damage stone and marble sills, so the solution used on the glass should not go on them without a second thought.
- “The frame can wait.” Next to a dirty frame, even clean glass looks neglected, and water running off the frame dirties the glass as well.
- “At least in the sunshine I can see what I’m doing.” You can, but the solution dries before you can squeegee it off.
What lies beyond the glass
A window is only truly done when the frame, the casing, the track, the blinds and the insect screen are all covered too. These spots are typically missed in routine cleaning; we have collected the other places like these in the article on the hidden spots of deep cleaning . When we clean windows, we clean the blinds slat by slat on request, and take down, wash and refit the fly screens.
If the surface is large or high up
With this technique, you can clean a ground-floor window well at home. For shop windows, glass walls, upper-floor or gallery windows, however, the equipment and safety matter as much as the technique; we covered this in the article on window cleaning at height . We work inside and out, including frames and sills, with our own equipment and products. The test of the result is the same as at the start of the article: afternoon sunlight falling at an angle across the glass.