Scrubs star Zach Braff was mortified when he was taped beating up a kid who covered his new Porsche in paint during a prank for TV show Punk'd.

The actor unknowingly appeared on Ashton Kutcher's practical joke show and his violent reaction to the stunt, set up by friend and Scrubs co-star Donald Faison, had to be cut from the show.

He explains, "Donald thought this would be the perfect opportunity to have me Punk'd. So we go to a liquor store, keep in mind there are cameras hidden all over the liquor store, so he knows he's on Tv.

"(Then) we go outside and these little kids were spray painting the car. I think that my brand new car with, like, 100 miles on it has been ruined with spray paint graffiti.

"They choreographed the whole thing and I wasn't supposed to catch the kid, but I caught him... and I'm not a big fighter or anything, but, with the adrenaline and everything, I just started pummeling him.

"I didn't know and it was so dark... (but) part of the punking was that it was, like, a 12-year-old.

"It turns out it was fake spray paint. They edited (the fight) out, because apparently you're not supposed to punch people on Punk'd."