Woah, I thought mine were bad, I hope you sanded it down until you couldnt see ANY black marks on the bare metal, otherwise 6months time you will need to re-post this thread
I did mine when I got the car 2.5 years ago, now I have one small patch returning. I spend nearly a week doing them.
Waxoll/underbody seal your wheel arches (with the wheels removed of course) aswell...
what i've been doing on my R107 restoration is to rub it down to get rid of as much rust as possible then treat any remaining black spots (rust) with a rust remedy and then putting a coat of POR-15 over the top and then priming and painting.