Recycled cardboard was treated with: Rustoleum sprayed lacquer (clear matte), Mod Podge or aq-based polyurethane. The spray lacquer dries to touch on 1 hr, can be handled in 2-3 hrs and totally dry in 24 hrs. The Mod Podge dries in 30 minutes and the aq. polyurethane in about an hour. All were then coated with acrylic gesso and paint applied ( acrylics/ kaolin clay and liquid car wax or watercolor, car wax and kaolin clay). Observation: all three are fine, but the spray and Mod Podge seem better. This is good for artists not having canvas available but wanting something other than paper. This is also good for studies before going to canvas and for beginners such as children (Mod Podge avoiding spray lacquer) . Of course, the cardboard is not archival. Also good for artists wanting to make their own foundation and paints. I like two coats of spray lacquer for my purposes.
Cardboard sprayed with Rustoleum clear matte lacquer. Two coats within 1.0 hr. I leave 24 hrs to fully dry and then apply acrylic gesso before painting.