Hard water doesn’t just look bad — it bonds. Calcium carbonate and silica deposit onto glass, then chemically fuse with the silica in the glass itself. Over years, untreated hard-water spots become permanent etching that nothing short of professional polish can remove. 0 PPM water leaves nothing behind to bond with. No spots, no streaks, no etching, no detergent residue — even when the water dries on its own in summer sun. Most window cleaners use a hose and a squeegee. We use a filter rig that costs more than a used car.