A neighbor saw me washing one of my cars and came over to ask my advice on his.
It was a one year old BMW and had never been cared for other than a simple garden hose wash with cheapo car was soap on an old T-shirt. It had tons of swirls so I wasn't worried about using my most aggressive clay (Red Clay Magic) on it.
I had him bring it by and it was just about the roughest surface I have ever encountered.
I started with a good wash just to get the surface dirt and bird crap off of it.
Then I test clayed a spot with fine blue clay. It was still quite rough afterwards.
Then I tried the red aggressive clay and it was better but not perfect. So on the next spot I washed it with Iron-X first, then the red clay. It was almost perfect, but almost is not good enough.
So for my final test spot it was washed, gone over with the red aggressive clay, then washed with Iron-X, then gone over with red aggressive clay again. Finally it was perfectly smooth.
I can only assume that the Iron-X just could not get down to the actual paint surface unless some of the crud had been lifted out first with the clay. And I did look at the surface with a surface microscope and it was peppered with rusting rail dust.
After all that it was then time to correct the paint and teach the owner how to properly wash his car to keep from re-installing all the swirls.
Just remember that the aggressive clays WILL cause swirls and marring that MUST be polished out afterwards.
Only a long ignored and unprotected paint surface will need this extreme a solution.