With this article, we’re going to look at Marine plywood and MDO. What are the differences? What are they used for? And why would you choose one over the other?

We’ll give you the lowdown on how MDO is made, how it’s sometimes harder to work with than marine ply and that it can be a lot cheaper. In this article, you’ll find out which ply suites which project when we look at Marine Plywood vs MDO – differences explained and compared.

So, if you need help in deciding if MDO or marine ply is the right choice for your project, read on.

What is MDO?

Medium density overlay or MDO refers to the special layers of fiber between each layer of plywood. Normal grade ply is just thin layers of wood, called plies, glued together. Usually with the grains at angles to each other.

MDO layers
MDO refers to the special layers of fiber between each layer of plywood

MDO ply uses a thin layer of fiber resin to bond each layer together. They also finish the outside surfaces (sometimes both, sometimes only one depending on the quality) with this resin. This makes MDO much stronger and more durable than standard plywood, and suitable for all sorts of outdoor applications.

First established around fifty years ago, they originally made MDO for sign makers . They wanted a weatherproof board that would be easy to paint onto and would last a long time. This is why some still refer to MDO as signboard.

Often confused with MDF (medium density fiberboard), MDO is a true plywood that is stronger, more durable, and will last longer. Whereas MDF is made from layers of wood fibers, they make MDO, like other true plywood, from thin wood layers peeled from the tree.

What is Marine plywood?

Marine grade is a type of plywood that manufacturers make to a specified high standard. Each layer is hardwood and selected for containing few defects. But what makes marine ply is the manufacturing process.

Marine plywood
Marine Plywood is made of hardwood with minimal defects

The best quality boards come with a standardized mark from a recognized authority. Each layer uses a waterproof glue and there are no core gaps which helps prevent moisture getting in between the plies. Finally, the bonding is conducted under strict pressure and heat requirements.

Aren’t MDO and marine ply basically the same?

The short answer is, yes and no. Some people consider MDO as a form of lower grade marine ply, and they often rank it as such. The differences are in the quality of wood used and the type of bond. With marine ply having fewer defects and absolutely no internal gaps.

But they’re both waterproof, right?

Actually, no. Contrary to popular belief, neither marine ply nor MDO are waterproof. They both use waterproof glues, and MDO usually has a resin coating. But they do not treat the wood or add any rot inhibiting chemicals. If exposed to moisture over time, boards will absorb water.

That is why it is extremely important to have a good finish. No matter which type of ply you use, make sure you coat the wood correctly and are particularly careful with the cuts and edges.

The reason for the waterproof glues is to stop delamination. If moisture gets in between the layers, it means that they won’t separate. Something that’s pretty useful if you’ve used it to build your boat or stand out in the rain.

Comparing Marine Plywood vs MDO

Now that we’ve established what they are, let’s have a look at MDO and marine ply’s features.

It’s clear that MDO and marine ply we’re originally designed for specific uses. As such, their key features are distinct. But as DIY’ers have found more and more uses for them, the overlap of usage areas has grown bigger.

Let’s look at what makes them both stand out.

Price

Compared to standard ply, both MDO and marine are more expensive. But with higher quality woods, glues and manufacturing processes, it’s understandable.

Of the two, marine ply is more expensive and, in some cases, can be up to twice the price of MDO. This is one of the major reasons MDO has become so popular. Unless you really need marine ply’s superior features, MDO is a good choice.

Weight

Another important feature is weight. Plywood weights can vary considerably depending on the woods used. But in general a similar sheet of MDO is lighter than its marine equivalent.

This makes MDO the preferred choice for weight sensitive projects.

Finish

Because of the resin layer in MDO, it gives a very smooth finish and accepts paint easily. Meaning that, when all else is equal, paint will last longer on an MDO board. It was, after all, originally intended for sign painters.

Brush painting white paint over wood
MDO has a smooth finish which makes it easier for applying paint

The smooth finish on MDO means it’s a great choice for paint work. You’ll usually not need to spend a lot of extra time on preparing the surface. For most applications, a fine sanding will be enough.

Marine ply usually comes with a veneer finish. If you’re after that traditional wood look, it can be a good option. But a lot of manufacturers have different veneers, and you may need to shop around to find the one you want.

Flex

The fiber resin in MDO makes the boards less bendable than their marine ply equivalent. If you’re looking at projects that involve a lot of shaping, then marine ply might be the better option. As a key component of a lot of wooden boats, marine ply was designed to be molded.

Strength

Both MDO and marine ply are strong. And considerable stronger than the standard plywood intended for internal use. But where strength is your primary concern, then marine comes out on top.

The combination of top-grade woods (to be classed as marine ply, the plies must be a minimum of B grade), no internal voids, and the exacting manufacturing standards, makes marine ply extra strong.

Waterproof

We talked about how neither is technically waterproof. Water resistant is probably a better term. But even here there are differences.

Because they originally designed marine ply to spend a lot of its life in water, it’s slightly superior. In particular, the fact that no internal gaps exist in the layers, means it’s harder for water to get between them. With marine ply, delamination should never be an issue.

But the differences usually aren’t that important for most standard uses. As long as you properly coat the ply, and take care when treating the edges, then both MDO and marine will stand for many years and take the worst that weather can throw at them.

Indeed, even in boatbuilding, there are a lot of vessels that make extensive use of MDO.

Typical uses

Now we know what MDO and marine really are and have explained and compared the differences. But what are they typically used for?

MDO is used for all types of signage, exterior paneling, fascia and soffits. A lot of commercial users like its smooth surface and excellent strength for building concrete forms. And it’s often used indoors for bathrooms or kitchens. A lot of hobby and commercial furniture makers like MDO for its strength and durability.

Marine Ply is often used for boats, due to its top quality

Marine ply is, not surprisingly, often used in boat building. The top quality, resistance to delamination and extra strength make it perfect for all marine applications including hulls, docks and jetties. But a lot of artisans swear that nothing but marine ply is good enough, and if cost is not a consideration, they may be right.

Conclusion

That’s it. We’ve covered marine plywood vs MDO – differences explained and compared. Now you know what they are, what they’re best at, and what to use them for.

If you need the absolute best of the best, where quality is the only criteria, then marine ply can’t be beat. The wood grades, the manufacturing, and the lack of internal voids make marine ply the premium product.

But if you’re looking for something almost as good, except with a better price point, you can’t beat MDO.

Now it’s time to make your choice and get cracking on your next project. Whichever you choose, you can’t really go wrong. Good luck and happy building.