I painted my calipers using G2 caliper paint, and while the finish isn't perfect, it's about equivalent to that of the stock non-PP calipers, so I'm not too fussed about it. They'll be behind wheels and dirty anyway, so I doubt anyone but me will notice or care.
When I was painting them, I wished I had some sort of plug to prevent me from accidentally painting inside the cylinders, so my friend and I designed one in Solidworks that fits perfectly with a bit of electrical tape to stop it from coming loose during the painting process and protect the cylinder.
Here's a link to the .stl if you'd like to print your own:
https://www.printables.com/model/217301-macan-brake-cylinder-plug
And some pictures
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