I guess this one's only for the crazy purists out there.....(like myself)
The XJ6 S1 was painted on the underside of the car, that is the bitumous undercoat was painted the same colour as the body.

Has anyone researched if this paint needs to be breathable and what type it is as i would
like to repaint it?

I have seen non breathable plastic type undercoats totally destroy modern cars with rust as they were non breathable and i would not wish to replicate that.
Regards
Peter