Thanks George, I have finally tracked down a pdf from the original manufacturer of magnesium (the UK company was called Elektron) at this link:
It talks about first pretreating the magnesium casting with a chromate conversion or hard anodic coating. (Never paint directly on magnesium)
then priming with paint containing not less than 15% by weight of chromate pigment. This should be either strontium chromate or strontium chromate containing up to 10% of barium chromate. Compounds of mercury and lead must not be used in the primer.
They don't mention zinc chromate or etch priming though.