First car show -- advice??

I know it seems like common sense not to touch someone's car, but I know you wouldn't say it if it didn't need to be said. Lots of ding dongs live among us.
Yup, at a show this past summer, I saw some guy just casually leaning back against my car chatting to the guy next to me. I got a lot of apologies when I asked him (nicely) not to lean on my car. Im gonna get some "do not touch" signs made up like I see alot of other people have.