Personalized Birthday Yard Sign Letters 18", Custom Name Age Balloons Birthday Cake Yard Cutouts Happy Birthday Yard Décor With Metal Stakes. Economical and durable, our Coroplast Yard Signs are a staple of outdoor advertising. These outdoor Birthday Outdoor Lawn Letters Decorations Yard Signs full color direct printed graphics are vertically fluted for use with a Stakes. Coroplast is typically used for yard, parking, real estate, and election signs. They are waterproof.