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Not just salt's the problem as winter washing and other wetness/moisture caught inside seams/welds etc can freeze/expand countless times throughout a winter season of above/below freezing temps w/ gap between growing larger each cycle.
More concerning than sightly rust may be today's unibody cars which although they passed structural strength crash tests when new, may not after 10 or whatever years of normal driving use in the rust belt?
~ Pete
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