I am not an expert on the subject. I never paid attention to mag wheels prior to coming here. Based on what I've read and the conversations I've had over the last 15 years, it is my understanding that paint applied directly to magnesium shall eventually bubble and flake-off. A chromate conversion coating is an electro-chemical applied coating we are all familiar with. Tools and fasteners are often finished with chromate. The chromate conversion coating adheres to magnesium and in turn provides a suitable surface finish for primer paint to adhere to. The chromate conversion coating also prevents oxidation, and it is somewhat decorative. But due to the electro-chemical manner in which it is applied it provides a thin hard coating which does not chip-off like paint.
I assume De Tomaso mag wheels had a chromate conversion coating applied prior to being primed with zinc-chromate primer.
Dow 19 was a product that could spot coat areas where the chromate conversion coating had been damaged (curb rash). It left a brownish/goldish coating on the exposed magnesium. If Dow 19 is no longer available I think I would contact Marvic in Italy and ask them what they advise using in its place … and what primers to use as well.
I've also run across information on the net claiming that magnesium can be anodized, and that primer paint will adhere afterwards to the anodized surface.