Marine-grade plywood, often simply called marine plywood, is not what it's often claimed to be. That is, it's not waterproof. And more to the point for everyday outdoor projects, it's also not rot-resistant. What marine plywood really is is a good-quality, hardwood plywood made with waterproof glue. Better grades of it also tend to be lightweight, strong and virtually free of defects. All of these reasons explain why marine plywood is a popular choice for building boats and boat parts.
It's also good for outdoor furniture projects that will receive a good protective finish.
Not Like Pressure-Treated Wood
Marine plywood is not rot-resistant because it typically is not made with wood species that have a natural resistance to decay, such as redwood, cedar, ipe, Cyprus, etc. Marine plywood also is not pressure-treated to resist decay, like pressure-treated lumber and plywood are. This means that if marine plywood will be exposed to moisture, it must be protected by a good, water-resistant finish. If you need a material that can stand up to moisture without a protective finish, use pressure-treated lumber or plywood rated for the expected level of exposure.
Why Waterproof Glue?
The main benefit of the waterproof glue used in marine plywood is that if the plywood is exposed to moisture, or even very high humidity or boiling temperatures, the glue won't fail, and therefore the wood layers of the plywood won't delaminate.
This is an important design characteristic if you're building, say, a boat hull with plywood. If the hull's protective finish is damaged and the wood gets wet, the plywood will remain structurally intact.
Good Plywood Is Different
The plywood products sold by most big-box stores is junk compared to marine and other types of quality hardwood plywood.
Standard plywood is made with fewer, thicker layers and mostly contains softwood, like pine or Douglas fir. The interior layers also have voids, or holes, that you can't see until you cut into the plywood panel. Marine plywood is a type of hardwood plywood. True hardwood plywood is made with thin layers of all hardwood, which generally is stronger and harder and has finer grain than softwood. Plywood sold as "void-free" does not have voids hidden in the wood layers. As for the layers, a lot of thin layers is better than fewer thicker layers because thinner results in a stronger, denser panel with edges that cut and sand cleaner.