I painted my styrene helmet with the white flexible plastic paint from Lowes and bothed the job.
I used Prep-all Kleanstrip that I bought at Walmart in the auto department. Its a surface prep wax and grease remover. I had a can left over that I used on a car hood. It says it can be used on metal, plastic, fiberglass.
Used a few cloths and lots of rubbing, and it took off all the paint. It didn't seem to do any damage to the plastic at all.
Repainted with the same plastic paint and it came out great.