The mineral-ring mystery (and how to stop it)
About half of my ridge-top clients share the same complaint: they wash the car, it looks great for twenty minutes, and then dries to a fine lace of white rings that won't rinse off with more water. This is hard-water spotting, and it's one of the most misunderstood problems in driveway car care.
What's actually happening
Tap water — especially well water up here, where it's drawing through a lot of granite — carries dissolved minerals. When a droplet lands on a warm panel and evaporates, the water leaves, but the minerals don't. What you're seeing in that white ring is a tiny circle of calcium and magnesium salts, etched faintly into your clear coat at the edge where the droplet held on longest. Wiping them with a towel just smears them. Rinsing with more of the same water just adds more droplets.
The three things that help
1. Wash early. The single biggest factor is panel temperature. A 70°F panel can hold a droplet long enough for you to dry it by hand. A 110°F panel will flash-dry it before you get the hose wound up. At the ridge-top houses I'm on the driveway by 6:45 a.m. in summer. That one change solves most of the problem.
2. Work in sections. Wash, rinse, and dry one panel at a time. Don't let any panel sit wet while you move on to the next. This is the opposite of what feels efficient, but it's how you beat the clock.
3. Final rinse with deionized water. I carry a five-gallon deionized-water jug for the last rinse on problem houses. DI water has had its minerals stripped; it can dry on its own, in full sun, and leave nothing behind. You can get a small inline DI filter for a home hose for under two hundred dollars. If you live somewhere with bad water, it's the best money you can spend.
If the damage is already there
Light mineral etching will often come off with a dedicated water-spot remover (usually a mild acid, used carefully, on a cool panel, in shade). Deeper etching needs a one-step machine polish to level the clear coat around it. Very deep etching is sometimes permanent. The ones I've seen that couldn't be fully removed were always on dark cars that had been sprinkled by an automatic lawn irrigator, daily, for years. Sprinklers are the single worst enemy of a car's finish in my experience. If yours overlaps where you park, move the car, or aim the sprinkler. That's the whole post, really.
— Rosa