Coating Plywood and OSB
Plywood and oriented strand board are common coating substrates for decks, trailers, truck beds, tanks, sets and floors. Moisture control, edges and joints decide how well coatings perform.
Plywood and oriented strand board (OSB) are engineered wood panels widely used for floors, decks, walls, roofs, trailers, truck beds, play structures, theater sets and secondary containment. Both can be coated with paints, sealers, epoxies and spray elastomers, but they behave differently from solid wood and concrete. They absorb moisture, swell, shrink and move at joints, and their surfaces contain resins, wax and veneer patches that affect adhesion. Good results depend on panel selection, moisture control, edge sealing and joint treatment. See coating wood for general wood principles.
Panel types
Plywood
Made from thin wood veneers glued with grain directions alternating. Grades describe face quality, for example sanded faces for painting and rougher faces for sheathing. Exterior-grade plywood uses water-resistant adhesives. Faces may have patches, knots and checks that can telegraph through coatings.
Overlaid plywood
Medium-density overlay (MDO) has a resin-impregnated paper face designed for painting, and is used for signs and exterior panels. High-density overlay (HDO) has a harder, smoother face used for concrete forms; it is more difficult to coat.
OSB
Made from wood strands bonded with resin and wax and pressed into panels. OSB is economical and consistent but has a textured surface and edges that swell when wet. Some OSB has a surface treatment or overlay.
Marine plywood
Made with higher-quality veneers and few voids, used for boats and wet applications.
Moisture
Moisture is the main factor in coating performance:
- Measure moisture content with a wood moisture meter before coating; manufacturers usually state a maximum.
- Store panels dry and off the ground.
- Allow panels to acclimate to the site.
- Avoid coating panels that are wet from rain or condensation.
Coating only one face of a panel can lead to uneven moisture absorption and cupping. Sealing all faces and edges is often recommended.
Edges
Panel edges absorb moisture much faster than faces, especially on OSB. Swollen edges push up coatings and cause cracks at joints. Common practice:
- Seal edges with primer or the coating system before installation.
- Seal cut edges made on site.
- Protect edges from standing water.
Joints and fasteners
- Joints. Panels expand and contract with moisture. Rigid coatings tend to crack along joints. Leave recommended gaps, fill joints with compatible flexible sealants, and reinforce with tape or fabric where elastomeric systems allow (see telegraphing).
- Fasteners. Set screws or nails flush or slightly below the surface. Fill heads if the system calls for it. Use corrosion-resistant fasteners for wet service.
- Deflection. Bouncy floors crack coatings. Panels must be thick enough and well supported.
Surface preparation
- Remove dirt, oils, release agents and loose fibers.
- Lightly sand faces to remove mill glaze and raised grain.
- Fill voids, splits and patches as the system requires.
- Vacuum dust thoroughly.
- Prime with a product suited to the coating system and panel.
Coating systems
Paints and stains
Exterior latex and alkyd paints are common on siding and sheathing. MDO panels provide the best paint surfaces. Stains are used on textured exterior plywood (see wood deck and siding coatings).
Epoxy coatings
Used on floors and containment where chemical resistance is needed. Epoxies are rigid, so joint movement must be addressed (see epoxy coatings).
Elastomeric and spray polyurea
Spray polyurea and polyurethane elastomers are used on trailer floors, truck beds, decks, sets, play structures and containment. Their elongation helps bridge small joint movement, and fast cure allows quick return to service. Primers designed for wood help reduce outgassing and pinholes (see pinholes and outgassing). Thickness and fabric reinforcement over joints should follow the manufacturer’s details.
Clear sealers
Water-repellent sealers protect panels during construction but are not durable finishes.
Common problems
| Problem | Possible cause |
|---|---|
| Cracking at joints | Panel movement, rigid coating, no joint treatment |
| Blisters and pinholes | Moisture, outgassing from wood, no primer |
| Peeling | High moisture, contaminated surface, poor primer |
| Swollen edges | Unsealed edges, standing water |
| Grain or strand pattern showing | Thin coating, no filler or primer |
See coating blistering for more on moisture blistering.
Fire and code considerations
Coatings on panels in buildings may need to meet flame spread requirements under ASTM E84 flame spread testing. Fire-retardant-treated plywood may require compatible coatings. Check codes and manufacturer data.
Key terms
- OSB. Oriented strand board, made from wood strands bonded with resin.
- Plywood. Panels of cross-laminated wood veneers bonded with adhesive.
- Exterior grade. Panels bonded with moisture-resistant adhesive for wet exposure.
- Edge swell. Thickening of panel edges as they absorb water, which can crack coatings.
- Moisture content. The amount of water in wood, measured with a pin or pinless meter before coating.
Frequently asked questions
Can OSB be coated for exterior use?
Yes, with proper edge sealing and a suitable system, but OSB is more sensitive to moisture than plywood.
Do I need to prime plywood before polyurea?
Primers are usually recommended to seal the wood, reduce outgassing and improve adhesion. Follow the coating manufacturer.
Why does my coated plywood floor crack in straight lines?
The lines are usually panel joints moving with moisture or load.
Related entries
- Coating wood
- Wood deck and siding coatings
- Pinholes and outgassing
- Truck bed liners
- Telegraphing (new)
- Coating EPS foam (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
