My neighbours cat has also scratched my car, an i required a machine polish to remove.
I now have a couple of weaves sticking up on my roof, which i presume the cat has caused... So yes they do harm.
Also a car cover isnt practical as the car need to be clean to use them... So everytime i use the car i need to spend an hour to wash it then cover it, isnt practical, esp in our wet climate.
Citronella oil worked for me for a while... Then faded.
Will get an ultrasonic box and put it on my dash see of that helps.