A form liner is any sheet, layer, or plate of material attached directly to the inside face of forms to improve or alter the surface texture or quality of the finished concrete. Many materials are suitable for lining forms; plywood, hardboard, steel, and most of the basic forming materials may be used. Also paper, rubber, cardboard, fiberboard, and even concrete panels have on occasion served as liners. Some of the most widely used liners are ready-made of textured plastics. BACKING SUPPORT FOR LINERS Patterned liners are used where there is concern for improving visual impact. Don't risk losing that special effect with a backup that isn't strong and stiff enough. The backup sheathing should have little deflection under pressure of the concrete and should be accurately built so that the liners can be precisely positioned. WOOD AS A FORM LINER Wood grain appearance can be achieved by casting concrete against plywood panels which have been wire brushed or sandblasted to reveal the grain. Unfinished sheathing lumber can be used to produce a rough board-marked concrete. Tongue-and-groove boards may be spaced with a small gap between to accentuate fins. PLASTIC LINERS There are several liner materials including acrylonitrile butadiene styrene (ABS) and polyvinyl chloride (PVC) sheets that are stiff enough to be considered self-supporting. They are usually attached to sheathing by nails, staples, or screws. Laminated glass-fiber-reinforced plastics (GFRP) form liners are well suited to simulation of stone, brick, and wood textures as well as to fluted shapes. Rubbery (elastomeric) plastic liners made of urethane or hot-melt vinyl are flexible enough to be peeled away from cast concrete with undercut areas. Thus design possibilities are virtually limitless. Polystyrene foam liners are often referred to as disposable liners. The rigid plastic foams can be either cut or molded to create complex liners with overall patterns or one-of-a-kind original works of art.