Sand/Grind Until Bare
WARNING: This page is about a different car, the 2018 Porsche Macan. However, it is still accessible from the selected car via links, so may be relevant.
To improve adhesion, sand the bonding surfaces until bare using 150 to 180 grain sandpaper. The sanding effect can be improved by cross-sanding.
Only use abrasive products based on pure silicon carbide (SIC) for sanding/grinding down the bonding surface until bare.
Sanding bonding surfaces until bare