Hmmm. Armor All? That makes sense, because the picture was taken right near my door handle, and my door handle is plastic and so I would have definitely have used Armor All on it several times over the last 18 years I've owned the truck.

Apparently when you're dealing with Armor All, something a bit stronger than Mineral Spirits is needed, but what? Oh well, I'm done with the paint job, bit i have an old Volvo I may start painting next, so I may have a chance to do more experiments.