After several sample sheets is now certain:
Belton Special Granite Effect (spray can) has made the race.
Pre-paint a black tone, let dry, within 48 hours with the granite effect paint without intermediate sanding over paint.
1.5-2 spray cans are enough.
Let dry very well (in my case it was 2 days) because this spray gets a high layer thickness due to its structure and because it is a water-based paint which requires longer flash-off times without high air flow, which would only be given in a paint booth.
Then you can finish with 2-component matt clear lacquer.
The picture is before the matte clear coat
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