Anyone with a place in Lewes or Bethany Beach knows what the air down there does to metal. The grill rusts. The bike chain goes orange by August. Patio furniture screws turn to crumbs. A fence takes that same punishment every day of the year, plus nor’easter wind and wet sandy soil around every post. So what holds up? For most coastal Delaware yards, vinyl and aluminum are the safest bets, since neither one rusts…