The inevitable questions about plywood include „How do I pick a quality panel for my paintings?“ (assuming you’re looking for quality) and „How do I prepare the panels for painting?“
A panel good enough for an artist to use should have one side (call it A) whose surface is smooth and free of knots, voids, plugs, and seams. The reverse side (B) can be slightly less perfect – perhaps with some sanded plugs – but if A is good, that does not necessarily mean only slightly less good; there can be some very poor quality B sides out there!
Also inspect all four edges – there shouldn’t be any voids, and the cut edges should be clean and vertical to the face planes. If you’re planning on using a 6 mm (1/4″) panel, it will probably have softwood veneers – pine or luan. If you want to use a thicker panel of, say, 18 mm (3/4″), birch is probably the most accessible hardwood veneer you’ll find. Moreover, birch plywood can have a very tight, smooth surface without a prominent grain that’s easy to size and prime. The quality of birch plywood can vary from species to species, however, so give your panel a thorough inspection.
If you’re going to use a panel thinner than 18 mm, down to 6 mm, it will have to be braced in order to keep the panel in plane. Panels 30 x 30 cm down to about 15 x 15 cm need only perimeter bracing; panels 15 x 15 cm usually do not need bracing as long as you can equalize the face surface tensions by coating both sides of the panel with equal numbers of coatings. Panels larger than 30 x 30 cm will need perimeter bracing and cross-bracing. Cross-bracing should be applied every 30 cm in both directions (horizontal and vertical).
These „auxiliary supports,“ as I call the bracing, ought to be built as units separate from the panel, and then applied to the panel. The usual size for a stick is 5 x 10 cm (nominal 1″ x 2″), although for large panels (up to 2.4 x 1.2 m [8′ x 4′]), I’ve used 5 x 20 cm (1″ x 4″) sticks. Most artists use store bought 5 x 10 cm pine sticks, but colleagues who make these panels as a profession have found them to have a strong potential for warping; they prefer to cut their own 18 mm x 10 cm sticks from 18 mm thick plywood, which, because of the glue used in their manufacture and the nature of their construction, are less likely to warp.
The easiest joints for the average artist/woodworker to make, where one stick meets another, are lap joints. Where cross-braces cross each other, use double lap joints. The tools you need to make braces include a carpenter’s square, a back saw and a chisel. Cut the sticks, make the joints, and assemble the auxiliary support with yellow aliphatic wood glue used by carpenters; allow it to dry overnight. I’ve also used construction adhesive (in a caulking tube) for the job, as it dries more quickly. Once the frame is dry, it can be glued as a single unit to the back of your panel. For this, I recommend the yellow wood glue instead of the construction adhesive; it’s more durable. Lay the panel on the floor and apply heavy weights to its surface to ensure a good bond between the panel and bracing. Allow to cure overnight. If you have a good number of simple squeeze clamps you can clamp the panel and bracing together, and stand the contraption on edge to get it out of the way.
A rigid substrate is definitely recommended for painting in oil, but a lot of artists don’t like the smooth and hard surface of a panel – whether plywood or another product. You can apply a linen, cotton, paper, or museum board surface to the face of the panel to soften the surface a bit and give it some texture. I use an acrylic dispersion gel medium for this because it gives me a lot of open time to reposition the material.
You can cut the fabric to nearly the exact size of the panel (leave a little for trimming) or make it about 10 cm larger in both dimensions, so as to have enough extra fabric to wrap around the edge onto the back of the panel or bracing. This makes a nice edge for decorating, should you choose to do something like that. Paper can be wrapped too, but it’s harder to do because machine-made paper has a grain that resists against bending.
Then, coat the surface of the panel with the gel medium about 1 mm thick using a brush, a roller, or a spatula. Put the fabric on it and position it so it’s hanging evenly over all the edges. Smooth any wrinkles with your hands, working from the center outward towards the edges. Once you’re satisfied with the position of the material, use a rubber brayer to roll the fabric down, again working from the center towards the edges. This will get rid of any bubbles and also make a good bond between the material and the panel surface. Some of the medium will squeeze out at the edges. It’s useful to coat both surfaces before attaching the material and while you’re at it, you can coat the cut edges of the plywood for their protection as well.
If you’re trimming the material flush with the edges of the panel, do it now, before the gel medium dries. Use a single-edge razor or a utility knife.
If you’re folding the fabric over the edge, turn the object over and put it face-down on a clean surface. Paint the gel medium on the edges and a bit of the reverse of the panel, then pull the material up and over the edges and attach at the back. I like to make a neat corner with no bulges by cutting out a neat square of the fabric at each corner so that the seams meet along the corner – that makes framing much easier. Stand the panel on edge and let it dry overnight.
To prep the fabric for oil painting with an oil ground, size the fabric with an acrylic dispersion medium designed for the purpose, and then apply an oil ground – traditional lead white in oil, oil/alkyd with titanium white, stack process lead white oil ground. Be sure to follow the manufacturer’s instructions and health warnings if you’re using a ground with lead carbonate in it. Two thinned coats of primer/ground are better than one. Use odorless mineral spirits as the thinner and be sure you have superior ventilation or wear a respirator. Allow to dry until dry-to-touch, usually 3 – 7 days in a warm environment.
To prep the fabric for painting in acrylic dispersion paints, size it with an acrylic dispersion medium designed to prevent Support Induced Discoloration (SID). GOLDEN GAC 100 is a good one. Then apply two to four coatings of a high quality acrylic dispersion primer/ground.
If you want to paint on the uncovered plywood surface, it still needs to be sized. The wood surface is unevenly absorbent and a size will even that out. Then you can apply an oil ground – traditional lead white in oil, oil/alkyd with titanium white, stack process lead white oil ground. Follow the precautions as above, and use a minimum of two coatings.
If you want to use an acrylic dispersion primer/ground, size it with something like GOLDEN GAC 100 to prevent SID, and then two to four coatings of a high quality acrylic dispersion primer/ground.
Suppose you want to paint in egg tempera? Make a modern glue/chalk gesso and apply five to ten coats of it to the sized panel. As you are doing so, ponder the meaning of gesso: an Italian word meaning „plaster“ or „gypsum.“
The final item to consider is how to educate your clients about keeping your paintings. A label is a good instructor, containing every bit of information about the painting from the construction of the support, to the contents of the size, the ground, the paint, and any final surface coating, using brand names where possible. In addition, you’ll want to tell the client that the optimum environment for storage/display excludes ultraviolet light, and keeps the temperature up to 24°C (75°F) +/- 5°, with the Relative Humidity at about 45% +/- 15%. Most homes can achieve the temperature requirements, but it’s very expensive to meet the museum standard for RH, so don’t make too big a deal out of that. Above all, you want your clients to be able to enjoy your artwork for as long as they wish, right?
Michael Reid O’Halloran, 1975, Plywood in Hostile Environments: Physical Properties and Applications, American Plywood Association.
Handbook of Finnish Plywood, Finnish Forest Industries Federation, 2002, ISBN 952-9506-63-5 
Engineered Wood Products Association of Australasia.
Pro Woodworking Tips.com.
For the metric conversions, Dashboard on an iMac computer.
Joyce, Ernes. 1970. The Technique of Furniture Making. London: B. T. Batsford Limited.
Wikipedia, the free online encyclopedia
Look up „plywood“ in Wiktionary, the free dictionary.
Canadian Plywood Association – CANPLY
The Conservation and Art Materials Encyclopedia Online (www.cameo.mfa.org, a free resource from the Boston Museum of Fine Arts
The Art Materials Information and Education Association (www.amien.org), an unbiased source of information about art materials. AMIEN is a resource for artists dedicated to providing the most comprehensive, up-to-date, accurate, and factual information about artists‘ materials. AMIEN is a 501(c)3 non-profit, charitable organization.