The two real causes of streaks
First, residue. Spray cleaners contain surfactants and perfumes that have to go somewhere. Wiped around with a cloth or paper towel, they smear into a thin film that is invisible indoors and obvious the moment sunlight hits at an angle.
Second, drying too fast. Clean glass in direct sun and the solution evaporates before you have finished the pane, leaving the dissolved dirt exactly where it was.
What professionals actually use
- A squeegee with a fresh rubber blade. Blades are consumable and a nicked one draws a line down every pane.
- A microfibre or sleeve applicator for wetting the glass, not a spray bottle.
- Warm water with a very small amount of plain dish soap. Less than you think, roughly a teaspoon in a bucket.
- Purified or deionised water for exterior work, which dries with nothing left behind.
- A dry microfibre for edges and frames only, never for the main pane.
The technique, in order
Wet the whole pane with the applicator and loosen the dirt. Then squeegee in a single continuous pass, either straight down or in a horizontal S pattern, wiping the blade on a dry cloth after every pass. Overlap each pass by a few centimetres so you never leave an untouched strip. Finish by drying the perimeter with a dry cloth, not by going back over the middle.
Clean the frame before the glass
Tracks and sills hold grit and dead insects. If you clean the glass first, the first wet pass across a dirty sill drags that mess straight back onto the pane. Vacuum the track, wipe it out, then start on the glass.
Hard water stains are a different problem
White cloudy marks that will not wash off are mineral deposits, and if they have been there long enough they have etched into the glass surface. These need an acidic restoration treatment or, in bad cases, mechanical polishing. No amount of ordinary washing will fix them, and scrubbing harder only scratches the glass.
When to leave it to someone else
Anything above ground level is where the calculation changes. Most serious injuries from domestic window cleaning come from ladders on soft or uneven ground. Water-fed poles reach three storeys safely from the ground, which is exactly why professionals stopped using ladders for this work.