Paul McCartney had a confession to make backstage after he and the remaining members of Nirvana grabbed the Grammy for Best Rock Song for "Cut Me Some Slack": He had no idea with whom he was playing.

As part of the "Sound City" project, Dave Grohl has approached McCartney and suggested they get together and jam on a classic like "Long Tall Sally."  Instead, McCartney thought they should write something new in the three hours they had together. Grohl showed up with two friends, Nirvana's Pat Smear and Krist Novoselic.

"To tell you the truth, I didn’t kind of know who they were. Seriously," McCartney said backstage. "I knew who Dave was because I’d met him. He said, 'I’ve got a couple of friends coming along' and then during the session, I hear them talking, saying “We haven’t played since Nirvana’ and I find myself in the middle of a Nirvana reunion."

McCartney said writing with Grohl and Smear and Novoselic was “quicker” than writing the songs for his solo album, “New," and said it was an excuse for a play date for his wife and daughter Beatrice  to go play with Grohl’s kids. "And then we'd sneak out," he said.  He also credited his buddy, Johnny Depp, with getting him excited to head into the studio. "He’d just given this little cigar box guitar which I was wildly excited about so I took it along with my Hofner [bass] and I said, 'Which one should I play, guys?,' and Dave gave me a funny look."

"It was THE Hofner, man!," Grohl said in his own defense.