MDO vs Marine Plywood: What is the Difference?
Both medium-density overlay (MDO) plywood and marine plywood are water-resistant composite wood resin products that can be used in both interior and exterior applications.
They are often said to be “similar but different,” and this is very much true.
Strictly speaking, an MDO panel is composed of an underlying plywood panel, which has an exterior bond classification, plus an added medium-density overlay, or surface treatment, from which it derives its name.
While most readily available MDO panels do not feature an underlying marine panel, marine ply can be sealed with an MDO.
For this reason, there is not a strict difference between MDO panels on the one hand and marine plywood panels on the other; a particular panel could be marine plywood with a medium-density overlay.
That having been said, the most common and standard versions of marine ply and MDO differ in terms of their composition, strength, and application.
In this article, we will examine these differences, compare the physical and mechanical properties of both plywood products, and identify which panels are best suited for specific types of projects.
Production and Grading
Like all plywood products, MDO and marine plywood panels are produced from thin wood veneers that are stacked and laminated in an odd number of layers.
The direction of the grain of each ply is arranged at a 90-degree angle to that of the previous layer.
For the odd-numbered veneers, the grain runs the length of the panel while those of the even-numbered veneers run perpendicular.
By arranging the laminations in this manner, a plywood panel is structurally balanced around its core veneer, resulting in being very strong and durable.
The resin used to bond the veneers in exterior-grade plywood including MDO and marine ply is combined with a natural filler such as chalk or coconut husks. Once impregnated with resin, the laminas are pressed and heat-cured.
Each plywood panel is composed of surface veneers (those visible on the surface of the panel) and core veneers (those sandwiched between the surface veneers).
The surface that is intended to be displayed, painted, or stained is known as the face veneer and the opposite surface is called the back veneer.
All plywood veneers are classified in terms of four grades: A, B, C, and D, with A being the highest grade and D being the lowest.
Since a complete panel has two surfaces, it is classified using a combination of the two grades, with the first letter indicating the grade of the face veneer and the second indicating the grade of the back veneer (for example, AB or AC).
Whereas non-marine plywood veneers including those used in other exterior panels can be drawn from a wide variety of conifer and broadleaf timber stock, production standards require that marine plywood contain veneers only from Douglas fir or Western Larch.
Moreover, all veneers in marine-grade plywood must be of either grade A or B. Hence, a panel of marine plywood will have one of three possible grades: AA, AB, or BB.
Production standards for marine plywood are governed by British Standard 1088 (either BS 1088:2003 or BS 1088:2018). All leading manufacturers abide by BS 1088, regardless of their location, though China also offers a similar, independent standard, as do Australia and New Zealand.
BS 1088 requires that a panel and its parts meet certain quality standards to be classified as “marine” grade. These include:
- face veneers must be free of knot holes, and no more than six pin knots are permitted per square meter;
- core veneers cannot be greater than 4.8mm thick;
- there cannot be gaps or voids within the core veneers;
- the phenol-formaldehyde resin must pass WBP, or water boil proof testing (see Plywood Inspection).
While these production standards ensure that marine plywood is suitable for use in boat building and exterior applications, it is important to note that the production standards do not dictate that such panels are chemically treated to resist decay.
While marine ply can be pressure-treated, it need not be.
Once pressed and cured, the production of a marine plywood panel is completed by sanding the surface veneers or applying either a medium-density overlay (MDO) or high-density overlay (HDO).
MDO panels are produced by applying a natural fiber overlay that has been saturated with resin to an exterior-grade plywood panel.
The underlying exterior panel on which the impregnated overlay is applied most often has a grade of BB, with C-grade cores.
As noted in the introduction, a medium-density overlay can be applied to a marine plywood panel; however, most of the MDO panels that are readily available to consumers do not feature a marine-grade underlying panel.
The medium-density overlay that is applied to the plywood substrate consists of a natural fiber that has been impregnated with phenolic resin.
Whereas high-density overlays (HDO) must contain more than 45% phenolic resin, medium-density overlays have a minimum of 27% phenolic resin.
Two types of MDO panels are available, and they differ chiefly in terms of the proportion of resin contained in the surface treatment.
The first, MDO-Concrete Form panels, contain at least 34% resin in the overlay. The second, MDO-General grade panels, contain at least 27% resin.
Comparison of Physical Properties
Both marine plywood and general-grade MDO plywood panels are highly durable and ideal for exterior use.
They are designed to withstand moisture, and unlike other types of plywood, they will not easily warp or bend, even when subjected to water on a regular basis.
Because the wood fibers of the veneers that constitute plywood transverse each other, both marine plywood and MDO panels are extraordinarily strong.
Compared to solid wood boards, plywood has a much higher strength-to-weight ratio, which makes it ideal for structural applications.
The veneers that constitute MDO and marine plywood panels are bonded with waterproof glue, which resists delamination when exposed to water.
While both MDO and marine plywood panels are designed to withstand exposure to the elements, neither type will retain its structural integrity after prolonged direct submersion.
Like all plywood products, marine ply and MDO panels are intended to be surfaced with paints, stains, or so forth.
Properties of Marine Plywood
Compared to standard exterior plywood, marine ply has the added property of being “void-free”; in other words, there are no air pockets embedded within the core veneers.
While all exterior bond classified panels are manufactured using waterproof glue, authentic marine plywood can be composed of only the highest quality veneers.
Even the inner core veneers must be of grade A or B. Such veneers have no knot holes and only a small number of pin knots are permitted.
Consequently, marine ply is solid, lacking the voids that form when lower-grade veneers are laminated. This makes marine ply much more resilient to moisture impregnation than other forms of external plywood.
Like other forms of plywood, marine panels can be shaped and bent. This malleability is a key feature that makes them ideal for use in the construction of boat hulls (see below) and decorative architectural elements.
Properties of MDO
As mentioned above, MDO panels feature an underlying plywood panel, which is bonded with waterproof resin and meets the specifications for exterior use.
An MDO panel will thus have the minimal structural qualities of its underlying plywood, with the added benefit of being encased in a resin-impregnated outer layer.
It should be noted that, unless specifically applied to a marine plywood substrate, MDO panels are not necessarily “void-free.”
Indeed, some cheaper MDO panels contain C-grade core veneers, which means that the internal veneers can have knots up to 1.5 inches in diameter. Any such knots will result in small voids.
Unlike standard marine plywood panels, MDO panels have additional layers of fiber-impregnated resin. This makes MDO panels more rigid and less flexible than their marine counterparts.
As the name indicates, marine plywood is used in a variety of nautical and maritime applications.
While it cannot be subjected to constant submersion unless it is properly sealed (for example, with fiberglass), it is often used as a structural component in the non-submersed elements of docks and piers.
Owing to its structure, it takes screws and other types of fasteners well and thus provide a solid basis upon which to attach railings, tie-offs, and other fixtures.
Marine plywood is often used in the construction of boats. The strength and durability of the panels allow them to be used for both structural and surface applications.
Moreover, through careful manipulation, marine plywood can be bent and shaped without sacrificing structural integrity.
This malleability makes it particularly well-suited for use in the construction of hulls for smaller vessels, such as canoes, rowboats, and kayaks.
When used in hull design, marine ply is coated with fiberglass cloth and epoxy resin, which bonds to the surface of the plywood, creating a waterproof barrier while retaining the natural wood appearance.
Beyond these exterior applications, marine plywood can be used for interior projects, including counters and cabinetry.
However, there is little gained by using such high-grade plywood in comparison to other options, such as standard-grade MDO. Moreover, marine plywood costs much more than suitable alternatives.
MDO panels have innumerable possible uses. Owing to their rigidity and smooth surface, they are particularly well-suited for outdoor signage and road signs.
Within the construction industry, general-grade MDO panels are used for soffits and siding, and MDO panels with a higher percentage of phenolic resin are used to create concrete forms and molds.
Because MDO panels are coated with a solid overlay, they have the appearance of a single board, and the overlay takes paint extraordinarily well.
This makes them well-suited for use in cabinetry and the construction of shelves, countertops, and other interior applications.
Manipulation and Use
As with all forms of plywood, special consideration must be given when cutting marine or MDO plywood.
Compared to solid lumber, it is more difficult to cut, and if it is not sawed properly, the edges will splinter.
To avoid these problems, both marine and MDO plywood panels should be cut with a table saw that is equipped with a plywood blade.
The blade should be positioned so it is one to two inches above the top surface of the panel. This ensures that the teeth of the saw blade will strike the surface veneer or overlay straight on, rather than at an angle.
Both types of panels take screws easily and it usually is not necessary to predrill a pilot hole.
Selecting the Appropriate Panel
Both standard MDO and marine plywood panels offer extraordinary structural integrity, an excellent strength-to-weight ratio, and superior moisture resistance.
Like all plywood products, they are fire-resistant, take screws easily, and can be securely mounted. Moreover, should the need arise, a panel of either sort can be easily replaced by purchasing a new one of the same grade.
When deciding whether to use a standard MDO panel or marine plywood, there are a couple of key questions to consider.
First, does your project require the malleability of marine plywood or the rigidity of MDO? While MDO panels can be bent, it is more difficult than in the case of marine ply.
Likewise, although marine plywood panels are largely resistant to warping, the additional layers of resin-impregnated fibers that are characteristic of MDO panels make them extraordinarily rigid.
Second, what type of finish do you desire? Marine plywood can be stained and sealed, thereby retaining a natural wood aesthetic. This is the case even when coated with thin layers of fiberglass.
While marine ply can also be painted, there is no denying that MDO provides a far superior painting surface.
Of course, if you are simply in need of plywood to create concrete forms, MDO is the obvious choice; its surface leaves a smooth matte finish on concrete and there is no need to pay the additional cost associated with marine plywood.
Both standards (sanded surface) marine plywood panels and MDO plywood panels are well-suited for a wide variety of interior and exterior applications.
While they are not, technically speaking, distinct classes of products, since marine plywood panels can be coated with an MDO, it nevertheless remains the case that there are differences between the types of MDO and marine plywood panels that are typically available to consumers:
- Standard MDO panels can contain lower-grade interior plies, whereas marine panels contain only high-grade plies. Hence, MDO panels can contain hidden voids.
- The surface of a standard marine plywood panel is simply sanded; it does not feature an MDO. Hence, it lacks the additional layer of resin-impregnated fiber that gives MDO its characteristic rigidity.
Owing to the high-quality construction and materials used in their production, both MDO and marine plywood panels are more expensive than other exterior plywood panels.
Before choosing between either, determine whether the project you are setting out to complete requires this level of investment or if a more affordable exterior panel will be sufficient.