Painting in plastic is most of the times unsucessfull job. As Husabutt said you have to sand all the piece very well with water based sand paper or with something like Scothbrite, then clen everything with a silicone free cleaner, apply a good plastic primer like "Self Hetching Primer" from Berger Paints or other similar (this product is a 2 componente one and is mixed in equal parts, very well mixed). Then look for a good acrylic paint in a can, don't buy spray they are scrap. You have a good brand from Berger also, which is "Standox" for automotive apply, let it dry well sand it again and apply a skin of an acrylic dual component varnish from Standox which is used for car finish and for the metalic colors.
Remember you have to sand very, very well every milimeter of the piece, every corner, every hole.
After all with all that work and money spent in paints it's better you buy new plastics, from Acerbis or other one.
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