The conditions under which a tilt-up panel is cast make it possible to provide control in the creation of an attractive surface appearance. Gravity, at least at the casting stage, tends to be the builder's ally rather than his enemy. As a result many panel surface designs are within reach of the creative architect and builder. To achieve a decorative pattern the panels can be cast face down with a geometric design formed with wood strips. Foamed plastics can be used to provide special decorative effects. The foam must be a rigid type such as expanded polystyrene which is glued or tacked to the casting floor. Undercuts or concave pockets should be avoided because they are difficult to fill with concrete. Two coats of a good quality, full bodied form oil should be applied to the plastic in a continuous thick layer. A variety of patterns may be produced in panels and large tilt-up wall units by using plastic materials as form liners. Among these are vacuum-formed thermoplastics, fiberglass reinforced plastics, or plastics formed into shape by heat and pressure. Since plastic is quite rigid and its surface smooth, concrete cast in plastic form liners is smooth and uniform in color. Inasmuch as most plastics do not bond to concrete, parting agents frequently are not needed. Proper vibration is necessary to thoroughly compact and consolidate the concrete. If an exposed aggregate finish is desired, the panel is usually cast face up. The aggregate may be either seeded over the fresh concrete surface or hand placed in a wet mortar, depending on the aggregate size.