Ohhh absolutely don't try that in the sun! I try to get that done before 8am in the summer, or after 8pm in the evening...

When the car is cool to touch.
One more thing to note, ceramic coatings make drying and washing so much easier... However, they don't prevent water spots, they likely make them etch easier IF water stands and it's hot/sunny. So while it's great to protecting the clear coat, washing easier, etc... you'll freak the hell out if you get water spots because then you risk having to do another correction and coating if they are etched really bad. If I had hard water, I would probably stick to a nice ceramic sealant applied monthly just so I didn't have to worry about ruining a coating.