I am taking the original paint off and I noticed I can take off the paint and the grey primer with lacquer thinner ( makes sense ). Under the primer is a black layer that doesn't seem to respond much to the lacquer thinner. It does somewhat strip with the aircraft stripper. I am curious about what this layer is. Does anybody know ?.

I would say that the panel your working on might not be an original panel or they sprayed an epoxy primer. My original fender has the redish-brown primer that aircraft stripper doesn't seem too budge. I also have a GM replacement fender from the mid 80's that has the black stuff under the primer. Just a protective coating from the manufacturer.

I'm pretty sure this is the original hood, I purchased the car in 1976 and it was in new condition then. If someone replaced panels, they did a hell of a job. The black stuff was also under both fenders. Probably is some type of factory coating.

Hmm. I could be mistaken then. I always thought the original primer on these was a reddish-brown color, at least mine was. I'm sure there may be diveances though. No chance the car was repainted before '76? I know, highly unlikely but............?