Marine-grade plywood, exterior plywood suitable for use in both fresh and saltwater, often is used in boat construction. In the US, it is made of laminated layers of Douglas Fir or Western Larch, which sometimes is overlaid with sheets of cellulose fiber impregnated with a special type of resin known as thermosetting phenolic resin. Plywood testing specifications contained in the publication “Voluntary Product Standard PS 1-09 Structural Plywood” were initiated by APA — The Engineered Wood Association and adopted by the Office of Standards Services, National Institute of Standards and Technology. The APA stamp on your marine-grade plywood lets you know the product meets stringent testing standards and gives you assurance of quality.
Adhesive Bond Performance - Three tests are applied to samples cut to specified sizes from a minimum of 20 sheets of plywood. Cuts are made differently depending on the test. In the vacuum-pressure test, samples are put into a pressurized container which then is submerged in cold water. Set vacuum pressures are applied. Once the container is depressurized and the samples removed, they are tested for bond strength by a shear testing machine. The second test is the boiling test. Samples are boiled in water for four hours, dried for 20 hours, boiled again for four hours, and cooled in water. They then, while still wet, are subjected to tension loading by a shear testing machine. The sample must hold together without de-lamination with the same success of a dry sample that is subjected to tension loading. Heat also is used to test the performance of the marine plywood's adhesive bond. Samples are placed on stands that allow them tilt backward and be subjected to high temperatures from the flame of a Bunsen-type burner. Each sample is heated for 10 minutes or until a charred area appears on the back side, whichever happens first. Then bond lines are checked to see if the layers have started to come apart.
Panel Performance - Samples are tested for performance while wet as well as once they are fully dried after being soaked. They are tested by concentrated static, such as that which might occur through the use of a power tool, and by concentrated impact, such as a blow from some object, to see how well they hold up. Whole-panel samples are tested by applying a uniform load. They also are subjected to tests that determine full-panel bend strength and stiffness. Further tests are conducted to ensure the panels meet standards for shear strength. One such test that applies pressure to opposite ends of each panel to determine how much pressure the panel can take before it bends and fractures is called a shear-through-the-thickness strength test.
Tips - Look for the inscriptions "APA" and "PS 1-09." These show the panels were manufactured according to the above standards. Marine-grade plywood also carries the word "Marine" and should be classified as grade A-A, A-B, HDO or MDO. Marine-grade plywood often receives no chemical treatment to slow decay. If such treatment is desired, look for plywood that has been pressure-treated with a preservative.
Warnings - Imported plywood may not meet APA standards. For example, some foreign countries allow the use of non-durable wood in marine-grade plywood.