With the myriad ways to combine resins and wood into layers to create plywood, it is no wonder we can get confused when selecting plywood. These are the general groupings—exterior, interior, marine grade, etc.—but the possible permutations are endless and you’ll have to ask about the specific product specs to be certain that you are getting a product that meets your expectations. You are not going to find most of these at the big box store. Inquire at a commercial lumberyard and prepare to be confused.
Water resistance. Plywood water resistance ratings refer to the glue used to bond the laminates, not the rot resistance of the wood used. Most construction plywood is rated water resistant (WR or X) and is acceptable for exterior sheathing and roofing. But this does not mean it can withstand constant immersion. There are also higher ratings for water resistance, listed here from least to most water resistant—moisture resistant (MR), boiling water resistant (BWR) and boiling water proof (BWP). The minimum grade for woods used inside the boat is for MR; for deck and hull work, choose BWR or better.
Hardwood vs. Softwood. Most plywood are conifers, and although all conifers are considered soft woods for grading purposes, there are differences among species. Fir and larch are strong and hard, and are used in marine ply. Likewise, deciduous trees generally thought of as hardwoods can be quite soft. Poplar, for example, is soft and weak.
Surface grade. Softwood plies are graded for each side, A being best and D being worst. CDX (water resistant with rough finish and voids both sides) is common for sheathing homes and AB is a common interior grade. Hardwoods substitute a number, 1 through 4, for the backside grade; A1 is good furniture grade.
Surface finishes. MDO and HDO we have described. Developed for quality kitchen and bathroom cabinets, nearly all boat interiors are prepared from MR plywood with plastic laminates applied to one of both sides, plus matching edge tapes.
A finished surface means there is no varnishing to keep up, and the wood resists wear and dings very well. On the other hand, once these woods get dinged, chipped, or deeply scratched, you can’t just feather the damage with sanding and then revarnish. However, unless the boat is in charter service, it will take a long time for the wood to look shabby. At the last U.S. boat show, more than 90 percent of the wood cabinetry was built using plywood with a surfaced finish. Solid wood, even as trim, is becoming rare.
Aircraft and Marine plywood. Both of these types of plywood are extremely water resistant and made of many thin layers of wood. Aircraft plywood uses only hardwoods, for maximum heat resistance and strength. Marine grade focuses more on rot resistance, often using larch and Douglas fir.
Sanded plywood. Some plywood features a thin hardwood veneer that is sanded to a smooth finish so that it resembles solid wood—mahogany for example. While this type of plywood might look enticing for making a new cabinet in the head, check the grade and the moisture resistance rating. The interior plies and the bonding glue might not hold up well on a boat.
Flexible plywood. This is just what it sounds like. Made using thin plies, flexible plywood is handy for cabin linings and curved panels, but again, you need to be aware of moisture resistance ratings. If you get a leak over your new ceiling panel you will regret false economy.