Once the fiberglass has been completed and sanded, it is time for the body filler. Apply body filler to all areas needed- areas that are not as smooth as preferred. Once the body filler has dried, sand down until desired texture is achieved. You will start with a more coarse grit working down to a smooth sand paper. Start with 80 grit and move to 220, to 400, to 800 and finish with 1200 grit. Sand lightly as to make sure you don't over sand and check frequently. Wipe the subwoofer box down with tack rags and inspect the surfaces. If needed, repeat body filler and sanding process in particular spots needed.
Once the sanding is complete, you may now prepare for painting. Make sure the area is clear of debris and dust, clean are thoroughly to make sure nothing clings to your subwoofer box when painting. The temperature of the room or space you are using to paint in needs to be at minimum 70 degrees and use proper ventilation and face mask.