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Huh? I'm not familiar with that design. Or, are you talking about a plastic moulding or garnish that is held-in with plastic trim fasteners? Salt and sand can get trapped behind those, creating a great corrosion environment... Having said that, sand and rocks do pummel the rocker panels and without a smooth design (like on the Mercedes W123 cars) and very good, self healing elastomers, said areas rust fast. Incidentally, in the ultimate, those Mercedes sedans become rust buckets, when with time the elastomer/"duraguard" surface becomes hard and no longer compliant.This is one reason why I much prefer cars that use a plastic outer rocker panel. The metal outer rocker panel are one of the first things to start rusting.