Squeegee

A squeegee is a rubber blade in a handle. Drawn across wet glass, it removes essentially all of the water and whatever is suspended in it in one motion, leaving a surface that dries evenly with nothing left behind.

Why spray-and-wipe streaks

A cloth or paper towel does not remove the dirty solution from the glass — it redistributes it and absorbs some. What is left behind dries in place, which is the streak. Add lint from the towel and the film that many consumer sprays deliberately leave, and the result is a window that looks worse in direct sun than before it was cleaned.

The difference is not effort or product quality. It is that one method removes the solution and the other spreads it around.

What still causes streaks with a squeegee

A worn or nicked blade, which leaves a thin line of water on every pass. Solution that has picked up enough soil to be redepositing it. Working in direct sunlight, where the glass dries before the blade reaches it. Not wiping the blade between passes. And not detailing the edges, where water pools and then runs back onto the dry pane.

Screens and frames matter as much as the glass

Clean glass behind a dirty screen still reads as hazy from indoors, and water trapped in a dirty frame or track runs back down the pane as it dries. This is why a thorough job addresses the screen and the frame rather than just the visible glass, and why a window can look clean on the outside and disappointing from the inside.

Any of this is learnable if you want to clean your own ground-floor windows — the tools are inexpensive. The case for hiring someone is height and reach, not technique.