I knew a guy who worked on the assembly lines who had actually sprayed the black on core supports. He said it was one of the entry level jobs at the plant. He stood in a pit below the car and as the car went by he was to spray the core support with a pressure painter that had a wand like a weed sprayer. He said the spray pattern was very similar to a weed sprayer too, especially if the pressure was low, and he said it often was. Another interesting bit of information was that since this painting was a rather undesirable job, where paint fell down all over the person, if anyone called in sick one of the pit painters would gladly step up to the next available position, leaving additional work for the other pit painter. That is why some cars appear to only have a swipe or two of black on the core support. He also said if two people from the line called in sick it may have been feasible to have two pit painters upgrade to a different job and that would explain the fact that some cars seem to have never had the core support painted.