I think I know what you are talking about. It's just a tray or part of the bracing to keep water from going into the compartment.
I can't think of a way you can paint this through the the fins.
If you plan on painting it black, possibly you can buy some enamel and hardener and remove the deck lid and pour that between a couple fins and move the deck lid around to make it all consistent
Or if you are looking for show quality, cut out the bottom piece, prep and paint it off, tack weld it and mold it back in.
Or mask up each fin and spray through them.