The best method for mixing paste epoxies is to use a stiff knife and a flat piece of wood. The putties are mixed between the fingers. It is helpful to store the epoxy in a warm area prior to use. Art supply stores offer a variety of palette knives for applying paste. Epoxy bonds to many surfaces including hard plastics.
Plastic is a broad term and includes many types of polymers.
It is always best to spot test if in doubt. Epoxy cures fast enough that there is significant strength after 3 hours. At 70 F the working time is 15 minutes. It is still possible to reposition work up to 45 minutes. Uncured epoxy will clean up with soap and water, or denatured alcohol.