None of my car is metal. The front fenders are carbon fiber, the front and rear fascias(bumpers) are plastic, and the rest of the car is fiberglass composite. So, yes, you can.
AS Mark is clarifying, painted plastic is fine. It is the same paint that is on the metal. Assuming it is factory paint, it should be similar thickness.
Non painted plastic, however, would not do well with polishers.
