Medium-density overlay (MDO) plywood is a specialty plywood designed for heavy exterior use. Unlike medium-density fiberboard (MDF), which it is often confused with, it is a true plywood made from thin layers of wood, interspersed with layers of a special paper, with several unique characteristics that make it ideal for use in areas where it will be exposed to direct weather.

Plywood Manufacturing

Thin sheets of wood known as veneers, or “plies,” are stacked to create plywood panels. Starting from the bottom layer up, each veneer is sprayed with glue. The back and face, along with all odd-numbered layers, have their grain running lengthwise through the panel. Even-numbered layers are run widthwise, making the panel resistant to swelling, shrinkage and warping. The face of most plywood is the best layer, although hardwood, cabinet-grade plywoods are made with a good face and back.

Properties of MDO

MDO is an exterior grade plywood with a weather-resistant layer of resin-infused fiber that seals it from the elements. The overlay is 28 percent resin and resists water. MDO is used heavily in commercial sign making. MDO is made only with 100 percent waterproof glue. The back and face veneer are typically B grade veneers, with interior veneers of C grade wood. Most MDO is produced with a wood-colored overlay, but some manufacturers use a colored overlay to signify their particular brands.

Applications of MDO

The fiber-infused layer of MDO makes it especially easy to paint, and painted surfaces on MDO last up to three times as long as similarly painted plywood. It is used in all kinds of exterior panels, including fascia, soffit and siding. MDO panels can be embossed with wood grain patterns to replicate natural wood for siding, and are often grooved to simulate boards. It is also used as floor and side panels for trailers, and is considered rugged enough for the interior of railroad box cars.

Plywood Versus MDO

MDO has all of the characteristics of plywood with the exception of visible natural wood grain. MDO, similar to plywood, can be used unfinished in areas where it will not be exposed to the weather, but must be finished for exterior use. The surface of the MDO must be sanded or scrubbed with paint thinner to rough up the surface slightly for proper paint adhesion. A coat of primer should be applied before painting for best results. Both MDO and plywood have similar strength specifications. The resin coating of MDO makes it far more durable than standard plywood. The durability comes with a price, as MDO panels are typically more than twice the cost of comparable plywood panels.

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