A rain repellent is a treatment that makes glass hydrophobic: water no longer spreads into a film but gathers into beads and rolls off. Visibility in rain improves and the glass is easier to clean. Here is when it genuinely helps, and when it does little.
The effect starts at around 60–70 km/h
This is the main thing to know in advance. On the open road the airflow lifts the beads off the screen by itself, and you can almost stop using the wipers. In town, where speeds are lower and you stop constantly, the airflow is not enough — the beads stay on the glass and you still need the wipers.
So a rain repellent pays off most for people who cover a lot of motorway miles. For city-only driving it is more a convenience when cleaning the glass than something you see while moving.
What it does not replace
A rain repellent does not make up for worn wipers and does not turn hazy glass clear. If the windscreen is sandblasted from gravel and marked by worn blades, the treatment goes onto a damaged surface and lasts a shorter time.
Where to start
The glass has to be in good condition first. Glass polishing removes wiper scratches and haze, and after that the treatment goes on evenly and lasts longer.
How long it lasts
It depends on mileage, on how often the wipers run, on the washer fluid and on the season. In winter, with road salt and the blades working constantly, the coating wears down faster than in summer.
The point of doing this is not the spectacular beading you see in videos — it is clean glass and good visibility in the rain.
