Why do sprinkler water spots appear so quickly and stick so hard?

Sprinkler and well water is often much harder than tap water, so each droplet leaves a heavy mineral load as it evaporates. Repeated daily wetting builds those deposits up in layers, and sun on a dark car speeds the process. Sprinkler spots can bond and etch faster than ordinary water spots. Park out of range where possible, remove fresh spots promptly with a mineral-specific remover and avoid wiping them dry, which can scratch the paint with the deposits.