If you want to use acrylic or water-based paint on plastic, you have to sufficiently prep the items. Typically, that means first sanding the surface to create "grip" and then applying a primer. Once the primer is completely dry, you can coat it with the desired acrylic paint. Be prepared to apply more than one coat, as acrylics do not always provide as much coverage as oil-based paints, especially over a smooth surface such as plastic.