We have discussed this over on FABO. The material was like thinned down undercoat and sprayed from underneath with a wand similar to a weed sprayer. There is a member with a extreme low mile Barracuda that has over spray splatter on the underside of the hood but no coverage on the top surfaces of the radiator support. Most colors got painted except the darkest shades but even that was hit or miss. There have been dark colored and even black cars painted and some car colors not painted.
Here's a quote: "I knew a guy who did the core support paint back in the day. He said it was painted from a pit and they used a nozzle on a hose kind of like a weed sprayer with water thin flat black paint. It was a nasty and boring job so any time someone called in sick or other reason to be gone the pit painters were the first to fill the missing position which left one person doing double duty in the paint pit. That is why so many were sloppy. One guy had to do twice as many cars. Sometimes both pit painters would upgrade for a day and those cars could have gone through with no core support paint, if there wasn't a newby to fill in."
And a link:https://www.forabodiesonly.com/mopar/threads/core-support-blackout.95796/#post-1972245121
Here's a quote: "I knew a guy who did the core support paint back in the day. He said it was painted from a pit and they used a nozzle on a hose kind of like a weed sprayer with water thin flat black paint. It was a nasty and boring job so any time someone called in sick or other reason to be gone the pit painters were the first to fill the missing position which left one person doing double duty in the paint pit. That is why so many were sloppy. One guy had to do twice as many cars. Sometimes both pit painters would upgrade for a day and those cars could have gone through with no core support paint, if there wasn't a newby to fill in."
And a link:https://www.forabodiesonly.com/mopar/threads/core-support-blackout.95796/#post-1972245121