Using filler

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.