Hello!
Thanks for all the comments!
templar1099 - I think it definitely has to do something with the plastic under the paint, I just wish I knew more about what's happening here. And how to stop it!
Greg - no problem, if somebody asked me I'd probably start with similar asumptions.
Tony - all replies are heartly welcome here! I agree with you on the plastic. I've had a similar thing once when I took an old part, laying there for years, sanded one detail off and then tried to paint it - the contour of the old detail showed through like twenty thin layers of paint! That one finally disappeared, let's see how this one will go. I have also thought about some protective layer, but I'm afraid that Motip might at least partially get through and then we would see really bad cracking of the surface... But I'll try that one too.
Thanks again for the comments and have a nice day
Paweł