Interior Finishing

EWPAA Tested and Approved plywoods with A and B quality face veneers, appropriately sanded, have a surface designed to provide a satisfactory substrate for high quality finishing with interior finishes, stains and paints. The A quality face is suitable for clear finishing. The B quality face veneer is suitable for painting. Under normal protected interior applications type D or C bonded plywood will give long term durability and preservative treatment is not necessary.


Before finishing ensure 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 of plywood with a wide range of materials is readily achievable for interior applications. Best advice is to follow paint manufacturers directions.


The use of high gloss finishes and paints tends to highlight naturally occurring and manufacturing characteristics of plywood such as beat marks from sanding, knots and open defects in underlay veneers, grain variation and patches, and is therefore not usually recommended. Satin and matt finishes and paints give a high quality aesthetically pleasing surface and are recommended.


Single pack polyurethane clear in a satin or matt finish is recommended for most interior plywood surfaces such as wall panelling, furniture and fitments. A highgloss finish may be used on tabletops, however naturally occurring and manufacturing characteristics may be highlighted as noted above. This finish is extremely durable in the fully protected interior environment and is resistant to heat, alcohol and household chemicals. A recommended procedure for applying a single pack polyurethane satin or matt finish is:


(a)
Sand the plywood with a very fine sandpaper. Sand in the direction of the grain never across it.
(b)

Apply one coat of the polyurethane satin or matt finish by brush, not roller. Do not brush vigorously as this will cause bubbles. Simply lay the finish on the plywood surface.
(c)

Allow the finish to dry thoroughly. Ensure at least the minimum time recommended by the finish manufacturer has elapsed.
(d)
Rub down the surface with steelwool to remove the nibs.
(e)
Apply a second coat of finish.

An extremely high quality finish can be achieved with this method.


Two pack polyurethanes, which are normally highgloss, produce an exceptionally hard, durable coating for interior plywood surfaces where extreme resistance to abrasion or chemical action is required. Applications such as laboratory bench tops, school desks and decorative flooring require this type of hardwearing finish.


There are newly developed clear finishes formulated on acrylic polymer systems. They 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 such as floors and bench tops. These finishes have the advantage that brushes and equipment can be cleaned using water.


Oil based stains should not be used. The oil raises the grain, darkens any roughness and highlights swirls in face veneer, leaving an aesthetically displeasing 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 and should be consulted.


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


(a)


Use modern formulations of finish which incorporate ultra violet filters. Both single pack polyurethanes, sometimes called "pine finish" and acrylic based formulations are readily commercially available.
(b)

Surfaces such as Victorian ash and Tasmanian oak can be bleached prior to the application of a finish containing ultra violet filters.
(c)

Small portions of white pigment can be added to clear finishes which act as ultra violet absorbers.

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


Extract from the EWPAA Publication "Facts About Plywood", courtesy of the EWPAA.