It's in the name, it's on the vans, and it's the question we get asked most. The short answer: rain is the softest water nature makes, and soft water is what spotless glass wants. The long answer is this page.
Think about what a water spot actually is. When a drop of tap water dries on glass, the water leaves but the dissolved minerals stay: a little chalky ring of limescale. Multiply that by a whole window and you get the cloudy, streaky finish everyone recognises. The usual fix is detergent, which brings its own residue and its own problems.
Rain skips the problem at the source. It hasn't travelled through chalk and limestone the way mains water has, so it starts almost mineral-free. We collect it at our depot and finish the job: a sediment filter, a carbon filter, then de-ionisation, until what's left is about as close to pure water as you can get outside a laboratory.
Pure water is hungry: it lifts dirt off glass readily and rinses to nothing. Worked over your windows with a soft brush on a telescopic pole, it scrubs, rinses and dries spotless. No squeegee needed, no chemicals involved, and nothing left on the pane because there was nothing in the water to leave.
Hard, mineral-heavy, dries to spots and film. Needs detergent to work, and detergent needs rinsing.
Look good for a week, then the residue starts grabbing dust and the glass dulls faster than it should.
Nothing dissolved, nothing added, nothing left behind. The glass stays cleaner for longer because there's no film for dirt to stick to.
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