Some kayaks are hand-built from plywood or wood strips covered with fiberglass. Paneled domes are constructed of separately framed timbers covered in plywood. Transverse bulkheads were also compositely built-up with several species of timber, plywood, and balsa. The wood is used for musical instruments such as ukuleles, plywood and low-cost furniture. The wings' outer surfaces were also skinned with a strong, specially laminated plywood. The wood is also excellent for production of plywood. It is also used for veneer for plywood. The buttstock is manufactured from plywood laminates. They often use steel pallets, steel straps with notched seals, outdoor plywood, and plastic film. MDF is generally more dense than plywood. The face veneers are thicker than traditional cabinet grade plywood. In the 1920s boards made of plywood or planking called Hollowboards came into use. Spruce and plywood ribs were connected with gusset joints. Prominent in this role were Roddis Plywood and Veneer Manufacturing in Marshfield. Larger cases are further reinforced with additional plywood or sheet metal.