I would be shocked and amazed if surface rust is covered at all under warranty. Surface rust is structurally of zero concern. Perforation (rust holes) or scale/pitting would be structurally detrimental. Even your automobile warrantee specifically states that perforation is covered for a given period, but there is no warranty expressed or implied on an automobile/light truck beyond "good will" or "best practices" for typical surface corrosion that may be offered to you by the dealership. Legally you have no recourse for environmentally induced corrosion or coating failure outside of perforation.