There are many unique types of plywood in the market today. Besides standard plywood, there is a particular type called marine plywood. As the name suggests, this grade of plywood is made to handle applications where it may be exposed to moisture or submerged in water. Discovered in the 1930s, marine plywood was first invented when a new waterproof adhesive was formulated around the same time. In the earliest renditions, this grade of plywood was used to manufacture boats during World War II. Beyond that era, marine plywood has made a case for use in outdoor furniture, boat parts, and coastal homes. So, what exactly makes a sheet of plywood marine grade?
Standard Versus Marine Plywood
Standard plywood is made from either softwood or hardwood veneers, or thin sheets. These sheets of wood are then glued together to form a thicker sheet. Most common plywoods are made from cedar, pine, and spruce. The grain of each ply is rotated 90 degrees to create a strong product. For marine plywood, the main tree species used in creation are either Douglas fir or western Larch. These materials are preferred for their fine grain structure, durability, and moderate density. The plies in marine plywood are also typically thinner than that of standard plywood. Having more, thinner layers is better for structural integrity than fewer, thicker layers. These thinner plies also promote cleaner sanding, sawing, and machining through the layers.
Wood species are not the only unique characteristics of marine plywood. The adhesives used to make marine-grade plywood are waterproof, and resistant to heat. Typically, marine plywood uses either melamine or phenolic adhesives. These types of compounds make the entirety of the board very resilient to water in the sense that each ply will not split or give out. The adhesives are also tough against heat, so even boiling water cannot penetrate the bonding layers.
Marine plywood uses melamine or phenolic adhesives making them very resilient to water.
The Grading System
Different plywood comes in several grades. Standard plywood is sold in grades such as A, B, C, and D, with A being the best quality. Marine plywood follows the principles of these typical grading levels, with dual grading. Dual grades describe a sheet of plywood from both sides. For example, an A-B grade sheet of plywood has a clean, almost perfect side and a somewhat knottier, looser side. Quite often, a project involving plywood will only see critical conditions on one side, or only be seen from one side, so it is economically sound to invest the critical side in a higher quality veneer.
The grades that marine plywood comes in are A-A, A-B, B-B, and two other special grades, medium-density overlay and high-density overlay. The last two grades feature a resin-impregnated fiber layer that acts as a weatherproof layer. The high-density overlay has a higher resin percentage, resulting in a shinier, harder surface.
Marine Plywood Performance
Most marine plywood is not totally waterproof. The wood used to create it is not chemically treated, so it is not immune to rot or weathering. Without treatment, the outer ply may experience degradation, but with proper manufacturing, will not ingress past the first adhesive layer. A pressure treated wood or chemically treated wood serves a better job at rot protection than marine-grade. In fact, a pressure-treated marine plywood is available at your local hardware store, and is suitable for even saltwater applications. Nonetheless, you can always apply a protective sealant or poly coat to give marine plywood the ultimate protection against the elements.
There is also a distinction between marine plywood and exterior plywood. These both utilize the high-end phenolic adhesives that resist weather and boiling, however the ply quality is more strictly regulated for marine grade over exterior grade. This is due to the fact that exterior plywood is more than likely to be covered up, so appearance is not of utmost importance like it is with the visibility of marine plywood on a boat or piece of outdoor furniture.
Marine plywood is such a phenomenal plywood product. There are very few options that protect your project from water and heat as well as marine-grade plywood. Your next project will last for generations when you use this hardy grade of plywood.
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