The Without You hitmakers earned a reputation as rock wildmen with their drink-and-drug fuelled antics in the 1980s, and they have given the young Baby hitmaker some tips if he truly wants to shake off his child star past.
Bieber has been caught up in various legal issues in recent months - he has agreed to undergo anger management therapy following a Dui arrest in Miami, Florida, pleaded no contest to vandalism for throwing eggs at a neighbour's home in California, and is still facing prosecution in Canada over claims he attacked a limousine driver in December (13).
However, Sixx and Neil have advised Bieber to step up his hellraising if he really wants a bad boy image.
Sixx tells Q magazine, "Let's give him a little bit of credit. He started with the eggs, which was a little weak - a poor start. Then he got arrested in Miami for drag-racing and that was kinda cool - he was under the influence. Then he got arrested in Toronto and turned himself in. So he's getting better, but he has work to do."
Neil adds, "He has to beat someone with a chain, or something. Or get into a fight with a cop... The next thing he'll get arrested for is toilet-papering a house."