Good question. We're patching a vent hole in the roof (PO added a fantastic fan on the other end, so we don't need the old vent--probably--it's filled now, anyway) -- but it leaves a square hole in the front closetloo ceiling. The ceiling is a partial shell of fiberglass; the walls are wallboard/thin ply/something else. I LOVE the original, pretty white ceiling, it even has molded in "designs" like a texture and border kind of thing...but now that area will have to be dealt with.
I was thinking we could glass it over now, fair it, wash it, acetone wipe it, prime it, and once the walls go back in, paint only the closetloo ceiling. That way the rest would stay original.
Best to you; I'd use "marine primer" myself, VERY thinly, on a 1000-grit sanded surface that has been well-wiped after sanding with acetone or rubbing alcohol, and then roll (with a foam roller!) something like Rustoleum Marine White. In our area, Ace carries it, Lowe's can order it; boatyards may have it. We've already bought all the marine white our LOCAL Ace had in stock, so waiting for them to put another whole quart on the shelf. It's nice paint. If you need a whiter white, maybe even Rustoleum auto paint in white. We tend to go with what we can find really close; you can find plenty of other paints. The primer designed for fiberglass, though, is key.
Best to you. One does want these eggs to be pretty and nice inside! I know we do, as best as we can.