My car is in full sun all day, at work and at home, and it does get insanely hot. Wonder if that’s why it only became an issue as the weather got warmer.
I literally bought that today and cleaned them, it seemed to do a good job.
I checked the coolant and it is at normal levels. This has been going on for months, so even a small leak would likely have produced lower levels by now. Plus, not using the heat, so not sure how that would be causing the film to form.
Your idea is as good as any.