The black is the e-coat. You're right, that is where most of the rust prevention is and it is also where the lead and cadnium are. Don't sand it if you can avoid it.

Also, do not spray this outside. E-coat cannot handle any UV. I've seen e-coated car bodies left outside start to chaulk in 8 hours.

That checking would be really strange on an OEM finish. Was this car refinished before?

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2010 Toyota matrix

'93 500 SEL
Well, I sold it. Just got to be too much to keep up with.
First time I haven't owned a MB since 1983.

'12 Volvo S80 T6
Needed something that wasn't as hard to deal with as my bad addiction

'18 Mazda Miata
No more boring cars for everyday transport!