Medium Density Overlay, abbreviated as MDO, is a pre-surfaced plywood product developed to make highway signs, so it’s about as weatherproof as you can get. One side is coated with a heat-activated phenolic resin/paper layer to provide a smooth surface that will take paint well and prevent grain show-through.
MDO is extremely water resistant, and while it is more expensive than common exterior plywood, it can be 40 percent cheaper than marine plywood and is more widely available. Most big box stores carry it. At Lowes we found a 4-by-8 foot sheet of 3/8” exterior grade (not marine grade) plywood for $42 a sheet; the same sheet in MDO sold for $72. For this report, we’ll describe the basic characteristics of MDO. Keep in mind that the specifications vary slightly between product lines and manufacturers.
MDO should not be confused with MDF (Medium Density Fiberboard), which is made from recycled wood bonded with resin. MDF is cheap and easy to work with, but weak and greatly affected by moisture. It has no marine use and does not belong on boats.
Pre-surfaced plywood is also available as HDO (High Density Overlay), which has a thicker resin/paper layer and higher levels of resin throughout the sheet. Although HDO is even more indestructible than MDO, the surface tends to be slick, and paint and epoxy won’t bond to it. It is popular for concrete forms; it is extremely durable, leaves a practically polished finish, and nothing sticks to it. With more decorative finishes, it finds broad use in furniture. The only boat building use is for cabin furniture.
Compared to marine plywood, MDO is 10 to 20 percent lighter. It takes paint better, with no sanding and the grain will not show through. For maximum weather resistance, you should seal the end grain with varnish or epoxy, but there is no need to sheath the surface in fiberglass. Need proof of its viability for marine use? Chris Craft used MDO in the 50s and 60s for its lapstrake Sea Skiff line of boats, and many of these are still plying the Great Lakes in the hands of loving enthusiasts.
MDO is a bit stiffer than marine ply. Narrow strips can be bent to fit lapstrake boat, but bending larger interior panels will require more pre-bending than marine ply, which uses thinner plies. MDO internal plies must be Grade B or better with no voids. Both MDO and marine ply are typically made from Douglas fir or larch. These woods are extremely water resistant, but neither is truly waterproof. The edges and exposed wood must be sealed and coated for maximum longevity. We recommend varnish or paint.
The MDO resin can have surface contamination, similar to amine blush with epoxy paint. Woods treated with phenolic resins can be tricky to bond with epoxy. The trick is to clean the surface with solvent before sanding. Unless you want a smooth finish or plan to paint the panel, exterior plywood or marine ply is preferred for projects that require a lot of gluing, laminating, or bonding.
We’ve used MDO for inflatable tender floorboards, companionway boards, and interior hatch covers. It is also good for interior cabinetry, as is water resistant hardwood ply, and prefinished kitchen and bathroom plywood (see adjacent article). In each of these applications, the finish on MDO was a nice advantage. As a bonus, the panel was stronger and more water resistant than one made from basic exterior grade plywood.
We would not use MDO for a transom or bulkhead, and we don’t like plywood for core in general, because of the way it wicks water; for structural panels we prefer to use foam Coosa Board or just solid glass (see “Tough, Versatile Coosa Board Suitable for Various Projects,” PS February 2020).
