How professionals get streak-free glass

The strip washer

A sleeve of absorbent fabric on a T-bar handle, dipped in a bucket of water with a small amount of soap. It lifts dirt off the whole pane in a few passes and holds enough solution to keep the glass wet until the squeegee. Spray bottles can't keep a large slider wet in Florida sun; a strip washer can.

The squeegee

A rubber blade on a handle, pulled across the glass to remove the water and the dirt suspended in it. Technique matters: overlapping strokes, a consistent angle, the blade wiped between passes, and a fresh rubber edge. A nicked blade leaves a line on every stroke. This is where most DIY streaks come from.

Detailing

The squeegee can't reach the last quarter inch at the frame. A clean, dry microfiber detail cloth takes care of the edges and corners. Skipping this leaves a wet line at the frame that dries as a streak.

Frames, tracks and sills

Most 'the windows streaked after they left' complaints are water running out of a dirty frame or track onto clean glass. Cleaning the frame and wiping the sill prevents it. It's also why we include tracks and sills on every home clean.

Purified water

Tap water leaves minerals behind when it dries. Purified (deionized) water leaves nothing, which is why exterior glass cleaned with water-fed poles can be rinsed and left to dry without spotting. It's the standard for exterior and upper-story work.