Next time just soak it in a "soup" of GoJo for a couple of days, rubbing at it in as it soaks, and it will come right off. I did the same to all my rubber seals and not only does it help remove over spray, dirt and grim but makes the rubber soft and pliable as if was new. So those of you out there with old and hardened rubber around doors, windows and trunks just remove 'em and soak it good and rub it off and rub it in!
That is good info on Gojo and rubber parts. Thanks! I have used it to detail spark plug wires and underhood hoses.
