Perhaps I'm off base here, but did you try washing it again and dry it immediately before the water drops dry up? When I washed my black `97 Dakota if I let any water drops dry I had the same problem you describe. They came right off when I washed it again. Often I had to carefully wash/dry the truck in sections, especially the hood. In other words wash the hood, dry it immediately, then on to the next section. You have to start at the roof obviously.

As an aside the same thing happens with my dark green sea kayak. Once the water spots are dry it takes a good wash to get them off. Even a damp sponge won't do it.

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Al