I think the grill is pp but its been a long time, either way on that I would use an adpro and a coat of epoxy, just to be safe.
Flex agent?? If it was needed, I would make! Why? Because my cost to make it is almost nothing and would be a big profit margin for us but I sleep good telling every one that calls, any flex will work in our paint and if it makes you feel better fine but it is a waste of money. Been saying the same broken record for 20 years.