From what I've seen on my E53 (10 yrs in "rains almost every summer afternoon" FL), there's something more then just water at work here.

Those areas on my X still have the factory black coating.

That shield drains water, the front is higher then the rear when installed.

It looks like someone parked the X in salt water for a few months!

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!