Don't know if that would help or not. Some plastics secrete resins/oils that are embedded to help protect them for UV and such and that ooze gradually loosens the adhesive. Sealing from the top keeps out moisture, but does nothing about the ooze from behind. Is this plastic a surface you have had problems sticking things to before? Decals stick great on somethings and not well at all on others - until you have tried one it is hard to guess what will happen.
We've had some success using rubber cement or fabric glue to re-attach decals that have loosened up over time.