Iceberg indeed! Remember magnesium needs a conversion coating, period!
Choices are:
Dow #7- toxic as all get out
Sanchem- pour down the drain with no issues
Until you convert the surface EVERY coating will ultimately fail, there is no way around this unless you keep the car/shroud in an inert atmosphere.
This is basic industry standards, some who drive a couple thousand miles in perfect weather might be getting good results, but if you drive year round(me), the process accelerates. Also the auto industry considers a car that lives within 2-300 miles of a large body of water(Pacific Ocean) is in a hostile environment.
If you look at the surface closely, inside the circle is corrosion and every pit is old corrosion. All it takes is a pit on a grain boundary line and you get intergranular corrosion.
Not a metallurgist, but I have stayed in a Holiday Inn Express once.
https://www.intechopen.com/books/magnesium-alloys-selected-issue/corrosion-types-of-magnesium-alloys