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I'm not sure of how far you want to take this but painting or powder coating SS has a full set of issues. An organic coating like epoxy will crack from the tools used to tighten the fastener; every crack or flaw in the coating on carbon steel will just get worse and disbond the adjacent coating. What you are looking for is a passive coating that is very thin and hard which usually means a 'plating' type treatment. You can do a black oxide finish on some SS. If the black protective layer is damaged or scratched the SS will passivate in the presence of oxygen and not deteriorate further. You might check this source and see if they might have what you are looking for.
I have rebuilt many wheels before. little tip ALWAYS use NEW hardware and perf OEM bolts/washers/nuts. That last thing you want is for those bolts comeing lose on the track. I have seen it happen many times. most wheels get tq @ around 8ft lbs so i would check with porsche. Yes the tq pattern is the "star"pattern. another tip mark each one after you tq hand tight first.
Not the socket on the back, but did you use an allen driver for holding the bolt head?
If he used a 13mm socket the bolt is most likely a 8mm. All the bolts that I've so far found on these cars that use a 10mm socket (aside from one on the trans linkage) are M6 bolts, and all the ones that use a 13mm socket are M8.
I'm still polishing one rim. All four centers are done and cleared. I have new stainless steel hardware coming in tomorrow from Fastenal. I have since discovered that Wurth has something closer to stock for the hardware. I'll post final pics on the wheels on the car in about a week when it's all done.