Fiberglass is very old school, as they did not know that fiberglass was hydroscopic.
It will draw water into it and rust the metal contact surface faster then when left open.
Your best bet it to tape up the underside after applying your rust converter
pour in a water proof epoxy resine compound, the same I use for sealing tanks.see below.
And when dry paint on the underside in layers. Maybe apply a clear coat just to seal the edge of the resine, as its in your headliner, I would not sand or polish it for max strength
You can then sand it smooth an cover with your paint and it will last indefinite.
Hope this helps.