I use ¼” Baltic Birch plywood. This is furniture grade, 5 ply, plywood. Regular 3 ply lumber yard plywood will warp and does not have a suitable surface. Baltic Birch is readily available but you may have to search around a little to find a supplier. It usually comes in a 5’ x 5’ sheet and most places will not make more than one cut (just so you can fit it in the car.) You will need access to a table saw to cut it to the sizes you want.
Backs - I sand the panels and then apply a coat of Minwax Polyshades (polyurethane with stain in it). It is available in gloss or satin finish. I let it dry overnight, sand lightly with 220 grade sandpaper, and apply a second coat.
Painting Side - I either apply canvas OR oil primer.
Canvas - Sand first and then “size” the surface. I use 2 coats of rabbit skin glue (sanding lightly between coats) but you can also use dewaxed shellac or Elmers white glue thinned to a brushable consistency.
The canvas I use is LLDP (Double lead primed portrait linen) from New York Central Art Supply (www.nycentralart.com). Cut the canvas about ¼” larger all around. I apply an archival adhesive (Jade Adhesive from either New York Central or Jerry’s Artarama) to both the canvas and the board for even coating and good adhesion. I roll out any bubbles with an old printers roller and weight the panels with some heavy books for several hours. I then trim the excess canvas with a razor knife.
Primed - sand and size as above. I then brush on a coat of Winsor Newton Oil Primer. When thoroughly dry (at least overnight, sometimes longer) I sand and apply a second coat. Allow to dry and sand lightly to get rid of brush strokes and any irregularities.
This seems like a very labor intensive process but I usually do 15 or so at a time. Compared to the price of good quality birch panels, it’s a huge savings and I can control the quality.
Do it yourself painting panels