Hold on ... Follow all those cracks to see where they might lead. Voids can be hazardous and can result in more damage.
Consider taping the bottom and leading edges. I bet Jake would agree, based on his J boat experience. I offer this because I owned a C 24 until it went down in a house fire next door to the boat.
My neighbor had a composite shop just up the street from where I live. Jim Bauman has many references in the multihull and windsurfing worlds. He was one of the guys that helped launch production trimaran racing by assembling Randy's F 25 C, Yo! That is, he performed the joining of the deck to the hull and maintained the boat's balsa core until he moved to CA last year.
He would advise you to skin down the gelcoat, laminate carbon and glass with EPOXY, and use microlight to fill for smoothness.
Re gelcoat or paint to finish.
Try to stage a thorough approach because on larger boats insignificant matters will turn against you when you never see them coming, sometimes at a worse time.
Keep advised via Jake. Jim is sailing and climbing mountains and difficult, sometimes, to reach. What practices we used on cats may not apply to boats that live in the water at times.
Spoke with a couple of folks (owners and others) and the consensus was that this is not a high-load/shear area. The crack hasn't spread for quite some time, so I suspect there might have been some single event that might have formed the crack.
Based on Jake's thoughts (not the carbon stringer stuff... I'm an amateur), I ground down until there were no cracks/voids to see if there was anything really obvious causing this problem, and I couldn't find anything.
It did appear that there was no lateral reinforcement of the core material.. just the two laminated sides. Maybe a very thin glass layer in this axis..
So I grabbed some epoxy and glass weave and laid up about 3 successive layers to build up that area (extended about 2 inches top and bottom of cracked area). Slapped a bit of gelcoat on top (slightly off color which may help identify repaired area in case something else happens down the road).
Hope to get it in the water this weekend (fx ESE 12-15kt) to see if (1) the repaired area is stronger than surrounding area which may cause more cracks (2) how the gelcoat holds after being applied to 24 hr cured/sanded epoxy (120 grit) and (3) if anything else pops up.
I did NOT use the microlight (dangit) which would have helped ease the sanding frustration!
The other frustrating part was trying to keep the glass weave together when cutting such thin strips..
next project: remove & fair daggarboard. Might have to rebed mast base plate screws