Is it necessary to use a sealer when painting?

Bob for this just wash the plastic with a good washing product even dawn dish soap will do well. be sure to rinse it well. let it dry and just before painting it wipe the surface with some paint thinner (check that the krylon is compatible with paint thinner of a test material.) spray and let dry. the thinner works to create a better bond as well as remove oil remaining on the surface. remember 6 inches from the surface its important to do this as closer causes runs and blotchy finishes father away and the paint is allowed to absorb to much oxygen from the air and is weak bonding. remove your masking materials soon after your final coat.