information/FAQs

Finishing of exterior plywood

PAA Product Certified plywoods with S, A or B grade face veneer quality have a surface designed to provide a suitable substrate for a high quality finish when appropriately sanded. Structural plywoods normally possess a C or D quality face veneer that is not designed to provide a high quality paint substrate. Structural plywood is available with A and B grade face veneers on special order.

All fully exposed plywood must be surface finished to prevent mechanical surface breakdown such as surface checking or crazing. This is caused by weathering or repeated absorption and loss of moisture. The surface may be overlaid with medium density phenolic impregnated papers, painted or coated with water repellents.

Medium density overlaid plywoods may be painted with a range of oil and water based paint systems. The following systems have been shown to be satisfactory:

Oil based: 1 coat alkyd primer, 1 coat oil based undercoat, 2 coats oil based exterior gloss
Oil/water based: 1 coat alkyd primer, 2 coats acrylic topcoat
Acrylic Latex based; 1 coat acrylic stain blocking primer, and 2 coats acrylic latex exterior topcoat
Other rigid exterior paint systems such as pigmented epoxies.

Raw hardwood and pinus veneer faced plywoods should not be painted with oil based or rigid paint systems. Full exterior exposure trials have shown that 100 percent acrylic latex paint systems perform satisfactorily on non overlaid plywood.

The acrylic latex system is compatible with Ruply, CCA and ACQ preservatives. It can also be applied to LOSP treated plywood provided the solvent has had appropriate time to evaporate.

Clear finishes such as varnish are not recommended for exposed conditions if a lasting finish is required. If a natural look is desired, exterior water repellent stains are recommended.

Edge sealing exterior exposed plywood panels is good practice as it minimises moisture uptake through the panel end grain.

Sealing the unexposed surface of exposed plywood is not recommended. This practice has been found to increase the incidence of fungal attack by trapping moisture in the panel. Using preservative treated plywood controls fungal attack in exterior exposed plywood.

For non-overlaid plywood, sanding the surfaces clear of defects usually provides a good substrate. Jointed surfaces do not provide paint problems, but patches and plugs can cause paint failure.

Horizontal surfaces present a far greater hazard to paint breakdown and surface checking than vertical surfaces particularly if they are subject to traffic. The finishing requirement will vary depending on the life expectancy and quality requirements. Solutions may be to cover new plywood with an outdoor carpet, or to paint overlaid plywood with non-slip epoxy paints. Plywood with factory applied 'non-slip' overlays is available from a limited number of suppliers.

Reference: Plywood Association of Australia