What are the benefits of using a car rack with a rust-resistant design, and how does it impact the overall durability and longevity of the rack, especially when frequently transporting bikes in harsh weather conditions or near coastal areas where saltwater exposure is a concern? Does a rust-resistant design provide any significant weight savings or aerodynamic advantages compared to traditional car racks, and are there any notable differences in the types of materials used to achieve rust resistance, such as powder coating, galvanizing, or stainless steel? Can a rust-resistant car rack withstand the rigors of heavy use and frequent loading/unloading of bikes, and are there any specific maintenance or upkeep requirements to ensure the rack remains in good condition over time?