The trunk lid and tailgates are constructed in a manner similar to a door. Two sheets of metal that are pressed to shape and held together by a 'hemming fold' lap joint. This overlap may appear to be filled with mastic, but this mastic is, in most cases, applied after pressing. The function of the mastic is to stop the electropaint and top coats on the raw edges of the metal overlap from breaking down and initiate the first spots of corrosion. The best advice for combating the corrosion in those parts is to inject a thin film hydrophobic protective compound that will prevent any water accumulation or condensation. (reference)
Courtesy Rust Check Kingston
Sand trapped behind trim can wear away the protective paint coating and expose the steel to the environment and subsequent severe corrosion damage.