Hi Jochen,
I have used "Fitz" on plastic display cases with great success. A lot of plastic modelers use it to buff out plastic windshields. It is a liquid buffing compound, comes in a squeeze tube. Look up Fitz in modeling magazines and websites. I have never tried it on a focusing screen. It's very fine stuff though. Another remedy is old fashion toothpaste, it's also a mild abrasive. I'd follow the advise above on where to use it.
Good luck,
2bits