Brand new the casting would look almost bright aluminum silver, but within weeks it would be a magnesium light gray and as it got older it would get a darker gray.

Also the runners are a primer gray but in PC I think I can only get this in gloss. Would flat or semi gloss rattle can stick to the PC?

This is one engine I've done in bright silver.

Mark, no answer to your question but over on the 911s registry guys are using Cerakote on a lot of 911 mag. As usual there is a big thread about it. The resident guru mixes his own brew of two different Cerakote colors I think.

Sounds like about 3 hours of internet snooping if you are interested.

I've looked into to it a bit, interesting but my PC guy is 15 minutes away, very high quality, I don't even have to degrease the parts, one week turn around and I haven't found anyone that beats his price.

painting over powder coat is tricky re inlet runners.
its very smooth for paint to adhere to.
might be be possible to sand it down to roughen it up enough.
and a good etch primer to bind to it first before finish coat.
a paint specialist supplier who knows their stuff could advise.
but i'd say it can be done.

looking at my original inlet runners, i swear its semi gloss or very low sheen finish from factory. its old and still original paint i'm looking at. but i don't believe it is a flat paint.
- more a very low sheen.
colour is slightly bluish grey rather than a full grey.

finishing mag is always an interesting one.
i'm a believer in leaving it alone.

i understand porsche originally used a clear honey like wax sprayed on - possibly only on gearbox. it didn't last for ever.

i don't believe VW would have used anything on the fan shroud casting. it gets very hot after engine shut down which you would know. maybe even hotter than mag parts on other conventional water cooled engines after shut down. (almost like its a heat sink for the crankcase?). wax or gell protection as used by some people would melt or dry off very quickly. powder coat is as good as anything if you must have a "new" look. but paint or powder coat can never quite look like raw cast metal?

my mechanic has a view that says its best to leave mag parts on an engine and gearbox dirty and oil filmed, best corrosion protection. failing that to use spray wax (same as used in spraying body work cavities - but only on the gearbox).

the original magnesium alloy in these engines and gearboxes is a german alloy recipe.
i can't remember what metal is mixed in any more, but its in there and it inhibits corrosion. the germans had a lot of experience with this stuff, masters of metal.
so if you can put up with the discoloration, the surface corrosion/oxidation is generally not really a problem (at least on the fan shroud). ultimately they will corrode away, all metal does in the end, but it is going to be a long time in normal circumstances. we will all be dead.

I had one recently cad plated and it turned out well.

If you have an existing reference you're trying to match, Prismatic Powders will send you free swatches.