My white car (medium sized SUV - Toyota Rav4) was progressively getting covered in black spots. I've read through lots of blogs and there doesn't seem to be a consensus. However, I decided to try the easiest... I used white vinegar and warm water with a ratio of approx. 1:10. It worked a treat. It took a good couple of hours to wipe down the whole car. I think I will need to go over it again, especially in the corners and joins, but it looks 100% better.