I was seriously considering using a wash of thinner as suggested. However, I wanted to prepare four figure sets for painting while still on the sprues. A wash with some kind of thinner would have required a lot of thinner and moreover not be very friendly on the environment.
I opted to wash the kit parts in the bathroom sink with the detergent we normally use for the floors, but in a quite heavy concentration (recommended dose is one capsule of ca 40 ml on ten liters of water - I used maybe three times as much in a sink with mayber two or three liters of water. The plastic parts were rinsed throrughly in this "soup" (which accidentally smelled of spring flowers instead of chemicals
) I was careful to make sure that all parts were thoroughly cleaned. But be careful - I broke a helmet from the sprues which now is useless.
After this "bath" the sprues were thoroughly rinsed in the shower, the water was shaken off and the sprues placed on the balcony to dry.
A test (albeit with a different colour) showed that the paint now sticks. The surface of the plastic parts is quite roughed up by the detergent and the fact that I had done what I should have done from the very beginning - just wash off the mould release agent...
I'll keep you posted on the figures.