If you can't get the silicone to stick to the plastic, try flame treating it first. No joke. Just make sure you don't start it on fire. Use a propane torch and just pass the flame over the affected area a few times. This will increase the surface tension of the plastic, and will help with adhesion. You can then let it cool and apply the stuff of your choice. Plastics generally have to be surface treated like this for printing, paint, or glues to stick.
Winkel