I have had a lot of luck with a product called " Goof Off " I do not know if it is available in Europe. I have a japanese helmet that was painted red.
I painted a small area 4 x 4 inches at a time letting it set 15 to 20 seconds ,starting at the top, and used a fine 00 size steel wool and with light pressure it came up. I had a little trouble with the inside corners of the star but with Q tips and patience it worked out. Keep excess wiped up with paper towels or a cotton cloth.
What ever you do make sure you do it in a well ventilated area unless you don't mind setting around drooling and stareing at the walls most of the time.

Plus what ever it does to your lungs.