The short answer
Wash with a little washing-up liquid in warm water, squeegee it off in overlapping strokes, and do it on a dull day rather than in sunshine.
- Brush the frame and sill first, or you will drag grit across the glass.
- Wash with a few drops of washing-up liquid in warm water. Not a lot: too much is what causes most streaks.
- Squeegee in overlapping strokes, wiping the blade with a cloth after each pass.
- Dry the edges with a microfibre cloth or a scrunched-up sheet of newspaper.
What will ruin it
Do not clean windows in direct sunshine. The water evaporates before you can get the squeegee to it and the dissolved dirt dries in place, which is exactly the streak you were trying to avoid.
Why it works
A streak is dissolved dirt and detergent left behind when water evaporates unevenly. The answer is less detergent, a clean blade and a slower drying rate, not more polishing.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.
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