I've found Sixt are far fairer than most others
I've used Sixt plenty of times and become a platinum member and had quite the opposite experience, once a small scratch on a bumper I knew had happened in a car park which was ignored and put down as general wear and tear.
Anothet time there was damage to the bumper and a resonable quote of £600 was given for which I paid half towards (50/50) which they then automatically provided me with all the documentation for so I could claim back on my excess insurance.
You can always find stories of people not being happy with damage to hire cars, my tip is to thoroughly check the car and take photos of the car on arrival and then you have the evidence if later needed that damage was there. If it wasn't then they're a business who wouldn't make money if they didn't charge you for damage, if you don't like that take the full excess out.
£600 does sound steep and I'd challenge why that can't be repaired for well under £100 but could be due to the colour/coating.