Opening laser weld seams: Procedure
WARNING: This page is about a different car, the 2013 BMW 550i GT, 2012 BMW 550i GT, 2011 BMW 550i GT, and 2010 BMW 550i GT. However, it is still accessible from the selected car via links, so may be relevant.
Roughly cut roof outer skin in the roof channel to improve accessibility.
Place angle grinder with grinding disc on laser weld seam and grind through top panel. Be careful not to cut through and damage panel underneath.
Remove sheet metal flange.