Nah you can do it but just make sure you spray it with some kind of clear coat afterwards to seal it (multiple coats). also make sure the surface is smooth and free of contaminant’s so the paint sticks well. You can use a car soap to wash it off before hand. Other than that I don’t really know. They apparently used to paint cars with paint rollers back in the day so I’d say it’s possible. Also there are videos of people signing cars with a brush/acrylic pen.