When buying plywood at your local supplier, you may notice that there are a number of different grades. The normal grading system uses the letters A, B, C & D, where A is the best quality, with virtually no blemishes and very well sanded. Grade D typically contains up to the maximum number of blemishes allowed.
The letter grades typically come in pairs, where one letter refers to the "better" side, called the face, and the other letter to the back side, opposite the face. As such, a sheet of A-C plywood will be very well finished on the face with a relatively unfinished back. Conversely, construction grade plywood would be C-D (commonly referred to as CDX plywood), which is great for structural use but not suited to be finish material.
Plywood Bonding Types
In addition to the plywood grades, there are four common plywood bonding types. The difference is in the glues that are used to bind the plies, or layers of the plywood. The type of plywood you choose will depend on what the material will be subjected to after the installation is complete. For instance, if a plywood will be exposed to considerable moisture, marine plywood would be the best choice. However, for kitchen cabinets, interior plywood would be fine, as there's no reason to spend the extra money that plywoods with waterproof glue would cost.
Plywoods for interior use only are made from various hardwood and softwood species and can be used only in interior applications such as wall sheathing, furniture (where exposure to moisture is limited), cabinetry and the like. Interior plywood is available in most grades, as well as a number of hardwood species such as birch, oak, and cherry.
For cabinetry projects and other plans that require one side of the plywood to be visible to the user, sanded plywood would be the best choice. These are typically sold as A-C plywood, with one side sanded to A grade while the interior side, which will not be seen by the user would be a lesser C grade. Most hardwood plywoods are sanded A-C grades, but some SPF varieties are also sold as one-side sanded.
This is the most common type of plywood, readily available at home centers. The glues used in exterior plywoods are much more resistant to moisture than interior plywoods. Once again, nearly all grades are available, with A-C, B-C, and CDX the most common. Numerous hardwood species are also available in exterior varieties.
When moisture resistance is a priority, look into marine plywood. This type uses the best adhesives and is manufactured to the highest standards. It also is most commonly graded as A-A, with two top grade faces, but is limited in the hardwood choices that are practical for use in marine settings. Additionally, marine plywood is considerably harder to find and more expensive than other grades of plywood.
When the appearance of the face is of lesser concern than the strength and stability of the material, structural plywood will typically be the choice. The resins used to adhere the plies are designed for extra strength to avoid separating the layers. Structural plywood is seldom found in a grade higher than C-D. It is commonly used in concrete forms on construction sites.