Seems to me that they might catch dirt etc. between them and the fender well or wherever they are mounted..... might be worse than not having them as far as rust goes.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k