Coatings Mil gauge

The Ferraris I've worked on (mostly 308's and 328's, couple of Dinos) all had a nasty thick layer of primer as their base. I mean really thick. Looked sort of like poly primer, IDK if it was. Using a gauge on those cars one would think there is plenty of clear or SS on there and get a big surprise when they burned through in an area or killed the paint cause they got it too thin.