If you have a boat or are planning on building something that will be submerged in water, then marine plywood is what you'll be working with. This type is a thicker material than regular plywood, comprised of about seven to 10 layers as opposed to three to five, with glue locking them all together. The adhesive in regular plywood will actually dissolve over time, causing the structure of the wood to break apart, but marine plywood is specially treated to withstand years in direct contact with water. By sealing this material, however, you can increase its overall lifespan. The information that follows will show you how to properly go about this project so you can get plenty of use out of this material for many years to come.
Sand the Surface
One of the things marine plywood has in common with other types is that you will always need to sand it down in order to stain, paint, or seal it. Go over both sides of the wood, as well as the edges, with 80-grit sandpaper by hand. Then, use a tack cloth to thoroughly clean up any dust you created during the process. You may even hose it down, but remember to allow it to dry completely before moving forward.
Apply the First Coat
Sealing marine plywood is a lot like sealing other types but it does differ slightly. In most cases, you would coat only one side and be done with it, but since marine plywood is going to be used underwater, everything has to be treated.
Dip a foam brush into the clear, penetrating epoxy. Saturate the foam brush, and start spreading it on the top and side edges first, switching to the front face of the wood when you're done. Allow the epoxy to dry then seal the bottom edge and the back of the plywood. This type of sealant, as its name indicates, works itself deep into the plywood and dries transparent. It also requires less maintenance than other kinds of epoxy sealer. You can use another marine-grade sealer if you like but you will need more coats and will have to reapply it every three or four years.
Add Subsequent Coats
It's a fact that plywood will rot regardless of what kind it is, but if marine plywood is not properly sealed with the right number of coats (especially if being used as a hull) it will deteriorate unexpectedly and cause serious problems for you. With normal plywood, you would use two coats of sealer, maybe even three. A good rule of thumb for sealing marine plywood is to apply as many layers of sealer as there are layers in the plywood. If you have purchased plywood with six layers, for example, apply six layers of epoxy to seal it. Between each coat, you add, gently sand to rough up the surface for better adherence of the next coat. Make sure you also use a tack cloth to remove dust before continuing to brush on more epoxy for best results.