So about that 300 I just picked up... What's up with this flaking? Hopefully just some kind of old coating that's separating out? Everything seems solid enough underneath, so maybe just wire it off and maybe POR?
The oils in the factory undercoat will evaporate out with time. A variable situation, no way to stop it either. Just flake off what is delaminating and use some spray can undercoat to patch the loss areas. Might take several mist coats. Of course, there's always black FlexSeal!
The grey stuff is the old undercoating, that chunk you flaked off after is rusty metal flaking off, I'd hit that whole area with a needler and clean it.
First a big heavy power wash with the strip tip on the Power washer wand. then drive around till dry, then have car sprayed with Krown or similar like rust preventer. If it is a concours restoration you will have much more work to do, but like John said do some touch ups after the power wash stripping and its dry, then the rust preventer. Fluid film thinned is good for your location.
FLUID FILM® is Eureka’s own unique lanolin-based brand of corrosion preventive and lubricant, used worldwide in a multitude of industries and applications.