I rented a house once that had the same problem on some huge windows in the living room, it drove me crazy. It was really bad. Near the bottom you could barely see anything through it. Utah water spots pretty bad too. When the old man who lived across the street saw me out front trying to clean the windows, he came over to tell me I was wasting my time, that others had tried, and they are permanently etched into the glass.
That made me all the more determined. I finally got it all off without damaging the windows. They glistened, there was no etching. I tried vinegar. (First I tried CLR, & got nowhere.) Then I tried no-scratch comet cleanser. I alternated between Comet and Vinegar, about 10 or so times each, until finally, there was nothing left. (Since the deposits were thinner near the top of the window, it was easy to tell I was making progress.)
There is bleach in the cleanser. You don't want to mix bleach and vinegar, so you need to rinse between each. Even after rinsing there is a slight amount of residual chlorine left from the cleanser. It seemed to react with the vinegar, actually foamed a bit. So when you rinse, you don't want to be 100% thorough. That seemed to be the trick. More deposits were coming loose with this technique than the CLR, vinegar alone, or comet cleanser alone.
I just remembered a new favorite product I've discovered since moving back to Utah. It's called "Shower Power". I'd tried it before, and wasn't impressed. Then one day, I read the directions.
It's very, extremely mild. It works by siting on the mineral deposits and breaks the molecular bonds. You spray it on, then do nothing. Seriously, don't even touch it till it's completely dry!
Come back after its dry and rinse the loose powder away. Repeat as necessary. It's basically doing the same thing as vinegar, but it's even milder. It works best when the mineral deposits are dry, when you apply it. (Probably because it's less diluted.) The beauty of this product is, you don't have to scrub hard or use abrasives, and it's very gentle. (It won't even hurt your skin) I've used it on gold plated bathroom fixtures, Italian tile, porcelain, and china. Some of my newer faucets are brushed nickle with a clear coat, and it hasn't damaged the finish on them, one bit. You get it at home depot. I'd try this before, the vinegar/cleanser method. You actually don't have to scrub at all. You might be tempted, but don't!
I can imagine that using any method that requires hard rubbing, or polishing is going to be extremely harsh on your paint.
In my experience, the solid paint colors are very forgiving, because they're nice and thick. If you have 30 year old metallic paint, the clear coat is probably very thin and weak. Your paint may not come through it, no matter how gentle your methods.
*Disclaimer: I am a Realtor, not a car detailer.
