There are some rain repellents that contain denatured alcohol as one of their ingredients. I'm no expert on materials chemistry but I've heard alcohol and rubber usually don't get along well together.

I've experienced the uneven effect of one rain repellent, where it leaves uneven patches of dry and wet windshield on every stroke of the wipers. The effect is a hazy windshield with inconsistent clarity. Needless to say, this was a headache to drive with.

If you're going to use a rain repellent, look for one that's alcohol-free.