If you want semi-gloss or flat white, just block sanding
until everything is uniform and painting a few times will do the trick unless there are some serious blemishes.
If you want it to look like gelcoat
with no waves:
Buy a long board to do the sanding
of flat surfaces. 3M makes nice ones, as is their sandpaper. Adhesive
backed, and comes in rolls.
spray paint on the ceiling that has a high contrast to the color currently there. This will show the low spots, where the color remains, and you need to add fairing compound. Where there are high spots, the cheap
spray paint will disappear.
Shoot various lines and zebra stripes over the area you are working, and replace as needed. I use cheap enamels, and spray from a decent distance away so the paint is practically dry when it hits.
I don't like sanding raw fiberglass
... so scuff up the low spots and apply a fairing filler. I see no reason why auto body fillers can't be used inside a boat, if they are the waterproof non-shrinking variety. (IE a step or two up from Bondo.)
For the curves you will need to sand them by hand. The easiest way to go about it, is to use a high build primer. Scuff the surface real well, spray it, and sand the primer till your low spots show themselves. I use a layer of rubber from a thick rubber industrial floor mat, fun noodles, and whatever else is soft and conforming. What you don't want is to grab the sheet of sand paper with your bare hands and rub low spots in with your fingers.