Finishing of interior plywood

Under normal protected interior conditions, type D or C bonded plywood will give long term durability without preservative treatment. Before finishing ensure that the plywood is dry (below 12-15 percent moisture content) and the surface to be painted is clean, smooth and wax free.

Clear finishing, french polishing, staining and painting with a wide range of materials are all appropriate for interior applications of plywood. Follow the coating manufacturer's directions.

The use of high gloss finishes tends to highlight characteristics of plywood such as beat marks from sanding, knots and open defects in underlay veneers, grain variation and patches, so is not usually recommended. A high gloss may be used on table tops.

Single pack polyurethane clear in a satin or matt finish is recommended for most interior plywood surfaces such as wall panelling, furniture and fitments. Satin and matt finishes are more forgiving of surface blemishes. In this application, single pack polyurethanes are extremely durable in the fully protected interior environment and are resistant to heat, alcohol and household chemicals. The following procedure for applying a single pack polyurethane satin or matt finish is recommended:

1. Sand the plywood with very fine sandpaper. Always sand in the direction of the grain;
2. Apply one coat of polyurethane satin or matt finish by brush. Do not brush rigorously as this will cause bubbles;
3. Allow the finish to dry thoroughly. Ensure that at least the minimum time recommended by the finish manufacturer has elapsed;
4. Rub down the surface with steel wool to remove the nibs; and
5. Apply a second coat of finish.

Two pack polyurethanes, which are normally high gloss, produce an exceptionally hard, durable coating for interior plywood surfaces where extreme resistance to abrasion or chemical action is required. They are appropriate for applications such as laboratory bench tops, school desks and decorative flooring.

Clear finishes formulated on acrylic polymer systems are fast drying and should not be rubbed down between coats. They are non yellowing due to the inclusion of ultraviolet filters but are intended for interior use only and should not be used on areas subjected to direct heat or extreme wear. These finishes have the advantage that brushes and equipment can be cleaned using water.

Traditional spirit based stains are deeply penetrating stains intended for use on darker species and give uneven results on pine, white woods, coachwood and Tasmanian oak. Oil based stains are recommended for these applications. Both these stains require a top coat of clear finish. A wide range of wood grain fillers and sanding sealers are available to assist in attaining a high quality finish on decorative plywood. Paint and finish manufacturers' literature is readily available.

Clear finishes on light coloured plywoods tend to yellow with age. Yellowing can be minimised by a number of methods:

1. Use modern finishes that incorporate ultra violet filters. Both single pack polyurethanes (sometimes called "pine finish") and acrylic based formulations are readily available.
2. Surfaces such as Victorian ash and Tasmanian oak can be bleached prior to application of a finish containing ultra violet filters.
3. Small portions of white pigment can be added to clear finishes. This acts as an ultra violet absorber.

It is good practice to experiment with samples to establish the final colour when staining or clear finishing plywood.

Reference: Plywood Association of Australia.

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Timber Research Unit
Department of Architecture
University of Tasmania