Just came back from the client's location.
So it's hard to say exactly what it is/was.
I started by washing the area with ONR. It didn't do anything (obviously)
I then sprayed with vinegar and let that work for a few minutes while I took all my gear and products out of the car (my van is in storage for winter... officially I am closed... hard to be a workaholic
I washed again with ONR. Did not see any change so I doubt it was/is salt or calcium
I then proceeded to do a tar decontamination with Tar-X. Again sprayed all the pannels and let work for a few minutes. When I wiped again with ONR, microfibers were turning brown, so definatelly a lot of tar there. It did not remove the white aspect of the pannel.
Clayed the pannels with Meguiar's Clay... Pulled out some more tar, it not seem to affect the white aspect but there was a log of contaminants present, very loud at first then dead silent
So I went to compound. Used D300 on a microfiber pad. Used both my GG 3'' and my Rupes 21. Rupes was not of much help because all the pannels we small and with a lot of character lines and curves. So I simply used it to do a D166 Cleaner Wax as a second step after the polishing, doing the actual compounding with the GG only.
So after about 10 passes of compounds, it did not remove all of it but it was a lot less noticable. I think it is damage in the paint from the pannels being peppered from debris from the very large wheels on the GL 350 (Mercedes). It looked like texture in the paint very hard to describe. Since this was an underground garage, I did not even try to take pictures, they never show much because of the very low light level, even if I put about 200W of light on them.
So I spent 3 hours on it, compounded the bottom of each of the 4 doors, lower fenders, and quick compound on the lower half of the doors so there would not been too great a change in texture from the very swirled paint to compounded lower parts. Then I did a quick D166 polishing on all those areas. I would say it looked about 75% better. The white could not longer be seen but if you went really close you could still see texture and a kind of road rash peppering we sometimes see on hoods and front bumpers.
Client was happy with the result. This is a lease and he is bringing it back in 3 weeks. Just wanted it to look decent. The rest of the paint was a swirled mess with a lot of deep scratches. I did not try to sell him on paint correction since the paint was extremelly hard and it would have taken an eternity to correct the whole huge SUV.
First time I see this kind of damage, not sure why that specific SUV has it and others don't seems to get it.