I've been stripping the underside of the pan on my 76 beetle and there seems to be a couple of layers. Obviously the dirt, then a thick black layer. There seems to be a layer between that and the bare metal that looks like a clear-ish lacquer layer. Am I imagining this or is it something vw really used? Maybe some kind of rust proofing? It could be quite important as I am about to use POR15 and it seems to stick best to bare metal.

Can anyone enlighten me?

Hi ...I am stripping the pans off my car and I cam repalcing them with new ones ...having said that i am using Rust Bullet as it is more flexible than POR 15 ...and you can paint over it!
My old pans are dead and there is two layers ...I woud suggest sandblsting the thing and recoating .... just an idea!
Cheers