Plywood resists warps, splits or cracks. It's available in large sheets that are consistent and square. It comes in almost any variety, and is easy to work with. Plywood is a major component in almost all building applications. But plywood is not an all-purpose material. Even though plywood is cheaper than solid wood, there are times when plywood might not be appropriate.
Layers and Quality
Plywood is made from real wood, with thin layers glued perpendicular to each other under hydraulic pressure and heat. The number of layers is directly related to the quality of the plywood. For example, Baltic birch plywood, one of the strongest types of plywood, typically has nine or more layers. Economy, or shop grade, birch plywood may only have four layers.
Plywood has a thin -- typically no thicker than about 1/32 inch -- veneer layer on both sides. Inner layers are typically made from fir or pine, but can be almost any type of scrap wood, but the veneer is peeled from a specific tree, to denote the type of plywood. For example, fir plywood has an outer layer of fir. Oak plywood has an outer veneer layer of oak.
Plywood is available in softwood or hardwood, just like solid wood. Softwood plywood, such as fir, is typically used structurally for sheathing, forms, roofing, shelving or anywhere large, strong pieces of wood are needed. Softwood plywood, because it's rough and considered somewhat bland, is not suitable for cabinets or interior woodworking in general.
Hardwood plywood is also used structurally, but typically only when aesthetics are involved such as in cabinets, furniture, paneling and woodworking. Hardwood plywood is more expensive than softwood, and used more sparingly than softwood.
Plywood is engineered to have an attractive face, as indicated by the veneer. However, the edges of all plywood types are rough, and somewhat ugly. It's typically not sufficient to sand them smooth, but instead, the edges must be covered with wood strips to beautify them. The ugly edge issue is typically confined to hardwood plywood, furniture and cabinetry, when aesthetics are a concern.
The thin plywood veneer on the face and back is typically glued on tight, but when cut, veneer can loosen. Loose veneer can also be a problem during sanding or when other procedures pull the thin veneer loose on finished projects, such as tabletops and furniture. If not glued back down, the veneer can pull loose in strips.
Working With Plywood
Guarded assembly techniques are needed when sanding or working with plywood. Amateur or inexperienced woodworkers often sand through the veneer, ruining the aesthetics of the plywood by exposing the raw core. The thin veneer can make plywood difficult to work with.
Plywood doesn't have the structural strength of natural wood. Screws strip plywood easily. Nails and other fasteners can bury too deep. Butt joints simply won't suffice in plywood. Only joints with splines, biscuits or other wood inserts are sufficient to join plywood.
Voids and Blows
Even high-dollar plywood can have defects such as voids and blows, which are hollow spots inside the plywood. Voids and blows are caused during manufacturing and typically go unnoticed until you cut the plywood. Voids may also take shape as a separation between layers, caused by bad or insufficient glue. If you cut into a void or separation, it's best to discard it, cut around it, or optionally plug it with putty or a wood splice.
Plywood is somewhat porous, and can shatter or splinter when cut. The thin veneer layer can be brittle -- particularly when the wrong saw blade is used -- and can shatter or splinter, and this often causes the veneer to lift. Always use high-quality cabinetmaker's or hollow-ground blades to cut plywood, and even then, there's no guarantee you won't get splintering.
Expansion and Swelling
Swelling can be a problem with plywood, particularly when fir plywood is used for sheathing and flooring in the presence of moisture. The failure to provide at least a 1/8-inch gap between sheets can result in buckling or warping of the plywood. The swelling of plywood, even when properly installed, is directly related to how much moisture the plywood is exposed to, and the duration of exposure.
The majority of plywood is not moisture-resistant. CDX plywood is an exception to the rule. CDX -- the letter "X," stands for exposure-grade -- is a structural, exterior plywood with water-resistant glue, and is typically used for exterior sheathing, flooring and anywhere moisture is present.
Face Grades, Veneer
Plywood veneer has a letter grading system. The letter "A' is premium-grade, "B," has a few defects, the letter "C," has more defects than B, and the "C" and "D" grades have more allowable defects. CDX exterior plywood has one "C" side, and one "D" side, making it somewhat rough and typically unsuitable for aesthetic purposes, unless you're going for a rustic look.
Plywood, more particularly hardwood plywood, can contain splicing errors. It won't show up until it's stained, but when veneer splices are not done correctly, a definite dark over light transition shows up in the finished product. It's typically too late when this happens, but the issue can be avoided by purchasing one-piece-face plywood that hasn't been spliced. It's more expensive, but is also more uniform and consistent.
Imported plywood is in a category all by itself, and there's a reason why it's so cheap. Also known as Chinese or Asian plywood, blondwood and whitewood, imported plywood may have foreign objects inside the core, thickness variations, peeling veneer, separations, voids and out-of-square continuity. Always check with the dealer before purchasing imported plywood. If you're not sure inspect the stamp on the plywood. It should have stamps from the American Plywood Association, Western Wood Products Association, or other United States-based inspection stamps if it's domestic plywood.