I think resting a hand on a car is probably a minor thing. Not worth the worry. Suntan lotion or no suntan lotion. If the suntan lotion makes a mark it's probably best the contemplate the transitory nature of things.So if someone scratched your pride and joy be it a car, bike, painting, pogo stick, colouring book (quite likely for some on this forum) or child's face, you'd be okay with that because it still functions fine?
One of the best lessons I ever received as a teen was having to repair/or pay for the repair of the damage I did to my mums car with the handlebars of my bike. Seems to me a few here could do with a similar lesson.
Then again, since the car in front of our house is fifteen years old and hasn't suffered from lotion, poo or bug abuse I'm probably pretty cheery about the whole paint thing.