Just picked up a '78 240D from the central valley in California. The tap water there is very hard and the car got sprayed by a sprinkler system. The PO let it sit in the sun rather than dealing with it immediately.
I've done some internet searches and tried vinegar w/ distilled water as well as a couple of Meguire's products. Nothing worked so I stopped by a reputable auto body shop for advice. The guy said that they were the worst spots he's ever seen and that color sanding was my only hope. He won't touch the job however since it's a 30 year old paint job.
A friend recommended 3M heavy duty rubbing compound. Anyone have any ideas? Thanks,

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'81 300SD