That's a new one. I've heard of armor-all, over used, building up in the grain of vinyl convertible tops, but...
For your situation I would suggest cleaning the tires with Wesley's Bleach-White. It is a strong cleaner so read and follow the label. It will remove the dull oxidized rubber. Dress the tires with Meguiars Endurance product. You can't do better than that.

dave and Xtoy - who says no to no touch

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