Stainless Steel Screws and/or Brass Fittings (when in contact with Aluminium) will corrode very quickly in damp and salty conditions.  For best results use Hot Dipped Galvanized Lag Bolts, Galvanized Coated Bolts or Nylon Isolators/Insulators to keep stainless steel or brass from contacting the sign board.  We don’t always have this hardware in stock for customers. The Isolators (Pictured Below) can be ordered from Spaenaur.  This is a Canadian Company and usually delivers in 1-3 business days. Find the catalogue here.  Alternatively, Hot Dipped Galvanized Lag Bolts can usually be found at Campbell River Fastenal or Home Depot.