I would still say remove the rubber grommets and plastic trim between side step and body first. It's not that difficult with plastic prying tool. Then clean it with Sonüs Aluminium restore. After cleaning protect the surface with nano sealant and push the grommets back on. You can use same sealant as on the plastic parts like Gtechniq C4.

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey