The best time to paint the insides of the wtertight compartments is now, before the deck or the deck framing goe on. The plans suggest shellac. I used a thin white paint instead. For models, I use acrylic artist colors and thin them very thin with water. I also used matte medium in this case, to dull the shine. … although who will be inspecting the paint through a tiny hatch?
The open part of the interior is finished with clear semi-gloss. I use Minwax Polycrylic.