I took a peek at your other questions. Everything, including this, are very typical behaviors for a kid with ADHD. They aren't trying to be mean or destructive, it is just cause and effect is fun, taking things apart if fun, figuring out what happens when.... You get the idea.
As a child I took apart just about every appliance my parents owned. I suppose this would have been destructive if I wasn't also able to put them right back together. Once I shattered a box of glass ornaments because I loved the sound they made when they broke. That I was breaking things or could break things really never entered my mind, that is ADHD. Broke assed executive function. You don't realize you did something wrong until someone is yelling at you.
Discipline has to bypass the executive function or higher reasoning. You have to make it be instinctive or a habit. Like breathing is instinctive you don't think, breathe, okay, breathe, okay breathe. You just do it.
So with the car talk to him. Well now we know what happens when you draw on a car with a stick. There is no eraser. Make him realize the experiment is over, we have results, no need to repeat. Oh yeah, we get bored easily. If an experiment will yield no new results, yawn. If there is no other stimulus he was getting from this project that will solve the problem.
If he got something else out of it you need to know and you need to find out in a, I don't think you are a freak, you are perfectly normal, but I need to know. Think about my ornaments, would you have accepted I liked the way it sounded? You need to because it is the only way you can teach him to control that impulse. Otherwise he will rationalize you do not understand and your words will have no value.
If he says it was neat the way the paint looked afterwards get him a sand box even if it is in a dish pan. A stick in sand looks the same but he can repeat that over and over without causing damage. He can use different sticks, fingers, your hair brush when you aren't looking. You have then satisfied his sensory need.
If you haven't noticed I didn't mention anything that even vaguely looks like discipline. You can't discipline these kids like normal kids. They are generally too smart and just get sneaky on you. You have to make them want to be gone, totally internal.
If I sound like a freak understand I was raised with traditional discipline, it doesn't work! I did whatever I wanted and my parents never knew. My kids were raised like I just described, all four ADHD, one with spectrum. Straight A students, always had the report card remarks, a joy to have in the class, wish I had more. The only thing I did was teach them to want to be good and to talk to me about experiments.
Oh yeah, and working off the damage does work as long as you don't make it so excessive that their thoughts go from this sucks to plotting how to never get caught again and having to do this.