that piece is notorious for rusting, the problem is that the bottom plastic panel traps water which cannot escape, best to paint it with POR15 then spray some body wax on it heavily. To aid water draining from the pan you can either use some thick plastic washers to create a space or make some holes.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)