Hey Dave, I'll help you out with the info.
Some background info on gelcoat surfaces. The cracks you have shown are not a structural issue. Fibreglass is somewhat flexible in nature and repeated movements/ weight in certain areas cause the harder brittle gelcoat surface on top to flex at different rates to the glass underneath. Impacts on the gunnel edges will do the same thing. The glass underneath will flex and the gelcoat cracks as a result.
The lay up of gelcoat in the factory is not a scientific approach. More of a "slap it on" approach in the 70's and 80's. Hand brushed was not uncommon. So the thicker the gelcoat is laid on the more chance it will crack as it has no reinforcing in the thickness.
2 pac poly is a very thin coating compared to gelcoat. And flexes quite well as a substrate naturally. Which is my more boat repairs are done in 2pac now days. Less time consuming after it's applied and can be glossy straight out of the gun.
If it was my project this is how I would tackle the paint job.
Yes fill all screw holes and chips. An epoxy filler is my choice because it's water proof. But car bog can be used as well. Any hole over 12-15mm should have a patch made from the reverse side. If it is in a panel that is 6mm thick I would grind it back to 2mm thick. Tapered grinding. A few circles of glass stepping up in size on the back will make it solid again. Then you only have a 2mm disc of bog. Again epoxy will hold on better than a car bog.
Counter sinking the holes is a good idea and provides better resistance to that cracked edge around the hole that can happen down the track.
Hair line cracks are inevitable. These as usually created when the deck is popped from the mould when the glass is still too green. The corners get stuck in the mould and air pressure or wedges are used to release it. Certain areas flex and cause these hairline cracks, even when they are new. It's not until years of use or dust can collect in these crack do they show up.
The correct way to repair a hairline crack is to put a small (4mm) drill into the end of each crack. Deep enough to hit solid glass. Then Drexel/die grind out each line 2/3mm wide it's full length back to glass only. Fillers can then be used to build back to level surface. If you don't grind them out you will normally find that over the years oil/wax/dirt sinks into those cracks. And when the primer coat goes over them you will get adhesion issues with the paint. Fish eyes etc.
Give everything a good 240gr sanding. Gelcoat is a good surface to lay paint on. I prefer JOTUN penguard high build primer. But any epoxy primer works well. Lay on 3 coats wet on wet. Then more sanding. 400gr wet is the best I find.
Now spray your preferred topcoat.
Long post but I hope it helps. If have any specific questions just ask.
Enjoy.
Damo's dodgy boat building factory.