It lives in the splash zone. Harbor side air carries salt year round, road brine joins it all winter, and the bottom section takes both, plus the standing water line where the door meets the driveway. The bottom seal traps that moisture against the steel. Where the door's model is still supported, replacing the bottom section and renewing the seal is the economical repair; where corrosion has climbed into the second section, the arithmetic changes and the quote should show why.
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