Once the primer/undercoat is dry, use filler/putty to fill the nail head holes. You can also take the opportunity to make good any imperfections in the ply wood grain using the filler almost like a thick paint. Once dry, sandpaper the filler to a flush finish with the ply surface. I use emery cloth in [lace of sandpaper. I find that it last longer and is more effective in this climate. Sandpaper tends to disintegrate in the humidity.