AFAIK, there is no exterior graded Luan sheet goods. It all has an interior compliant glue. As was stated, use an exterior grade 3/8" Plywood. It should be stamped "BCX" or similar. The first two letters are the grade of finish plies and the last denotes exterior grade. A surface filler is typically used on screw indentations and seams, that keeps the EPDM from resting into a crease or hole.