Since concrete is a plastic material in its initial stages and will reproduce the pattern against which it is cast, form liners have received considerable attention from architects in the past few years. Normal form materials are steel, plywood or plastic coated plywood. The plastic coated type gives a very smooth and water resistant finish to the wood, much the same as plastic liners or fiberglass. Other types of plywood with patterned markings- sandblasted or wire brushed- are used to bring out the grain. Still another type offers striation, or irregularly spaced grooves. Interesting effects may be created by preplanning the layout of forms or liners to give a stacked block, checker-board, or other varied design. There are many types of ties available, such as snap ties or ties with a breakback point. This type leaves a hole which must be patched. It is rather difficult to match the finished color of the. concrete, and quite often the holes are accented, rather than concealed. To make the operation a little easier, cones of varying depth can be used which leave a cleaner, chip-free hole to patch. Another aid is the use of waterseal tie, which has a washer in the middle of the tie that remains in the concrete wall. These ties help to prevent water leakage through the concrete wall where the tie is located. They are particularly useful in sewage treatment plants and swimming pools. The next consideration, regardless of material or finish, is the form coating or release agent to use. The basic use of a coating is to enable stripping of forms with as little difficult as possible and to prevent uneven coloring of the concrete. The selection and use should be closely controlled, as some coatings can stain the concrete surface.