The English dictionary describes the word porous as an object that has minute interstices through which liquid or air may pass. In simple English, this means a surface that has holes in it, will naturally absorb more paint and therefore a light coating of primer paint should be used before any acrylic painting ideas are thought of, to avoid any disappointment. The few porous items you should be aware of are untreated wood, certain cardboard, sponges, and fabrics.