… It talks about first pretreating the magnesium casting with a chromate conversion or hard anodic coating ...
... either strontium chromate or strontium chromate containing up to 10% of barium chromate ...
They don't mention zinc chromate or etch priming though.
The document "Surface Treatments for Magnesium Alloys in Aerospace & Defense" is a great document, thanks for providing the link. It seems that information regarding anodizing the magnesium part has some merit.
Strontium, barium, lead, and mercury are all quite poisonous. Those old primer recommendations are no longer viable (at least not in my world). If a chromate primer is needed to best adhere to a chromate conversion coating then perhaps anodizing would be the better "Modern" process to follow. By better I mean it would be less toxic and easier to acquire compatible paints for it. But can a part that has been treated with a chromate conversion be re-treated via anodizing? Probably not.
One thing that has not been explained well in this topic, but is clarified by that document is the simple fact that a zinc chromate primer does not repair the chromate conversion coating when it is scraped-off by curb rash, clip-on weights, etc. That's what Dow 19 was good for. After repairing damaged areas with Dow 19 then a "compatible primer" such as zinc chromate could be applied. I guess Dow 1 was the product used when the entire wheel was being re-coated with a replacement for the conversion coating.
By the way, members have referred to the yellow and green primers sold by "Tempo" as zinc chromate primers but they are not zinc chromate, they are zinc phosphate primers.
That still leaves us with the questions (1) how to repair a chromate conversion coating in the absence of Dow 19, and (2) what modern primers are best compatible with that conversion coating? Marvic is one possible business that may offer advice or guidance. Nate's approach has merit too (i.e. let an expert do it). Problem with that one is finding an expert.
Another possible solution would be a set of replica wheels made from mono-block aluminum billets. Although I write that jokingly, that is kinda where my head is at.