Hi Jason, to be honest, I have never thought about it.
I believe you are correct about the primer though. In the latter period of Scott Bonnar, prior to the merge into Rover-Scott Bonnar, (1968-1980) they were using a hammertone finish which hides a multitude of sins. To save money probably (and time of course), to my knowledge they didn't use any primer. To be fair though, most other manufacturers didn't use it either.
As for galvanising, I think this would be prohibitively expensive for most people and comes with its inherent set of problems. It would have to cure for some considerable time prior to painting; a gal primer would then have to be applied followed by a good quality zinc chromate primer then finally top-coated with what ever finish one would choose.
This method would result in a rust free Scotty, but would not reflect in dollars if ever you wished to sell the machine in future.
There are easier methods to achieve the same result.
