24"x18" Corrugated Plastic 4mm Sign Panels, Color=White, Vertical Flutes are 18" Long. Smoother or Flatter than most Corrugated Plastic on the market and these also let less light through the sign panel than most Corrugated Plastic on the market. With multiple sellers on this listing, make sure the coro you receive is the relatively smooth and opaque variety. While no corrugated plastic is smooth, Signs Direct's is smoother than most. While no corrugated plastic is opaque, Signs Direct's is more opaque than most. NOTE: Flutes should run vertically if used outdoors to allow for water drainage.