Unfortunately, I think that the hard way is going to be the right way. If there are only a few cracks,you could just sand back broad areas around the cracked gelcoat, right back to the glass & lay on a high-fill 2-pack poly filler, stacking it on the sanded areas so that you can block the whole surface back to perfection. If the cracks are everywhere, you can bet there are heaps more under the surface, waiting to show up just after you finish the final paint job !
If this is the case, it would probably be best to strip all the gelcoat off (air powered rotary & orbital sanders will do this fairly quickly) & then lay about 3mm of the filler over the whole car.
I have a '71 Vette under resto by a professional & he has tried all sorts of approaches over the years. He has finally settled on the Spies Hecker (DuPont company) system & is getting fantastic results.
Everything is 2-pack, starting with Raderal 3508 high-build polyester filler, then Vario Primer/Surfacer 8590, then Permacron 293/295 Base Coat & finally Vario Clear Coat 8000 or Permacron Clear Coat 8110.
After watching the "expert" do the 'Vettes, I have started to do the '40 myself - I have a couple of panels in Raderal & have blocked them back to 320 grade so far - easy to do & a great finish. All you need is a "putty gun" with a 2.5mm nozzle & away you go - you can't really put too much on & you don't need to worry about runs - it all blocks back beautifully, ready for the primer.
At that point, I'll let the "expert" do the base & clear coats (tried that once & really stuffed it up !).
What a lot of waffle !! I guess that the long & short of it is that you could do quick "dig-out & fill" job on the cracks, then prime & top coat, but the risk is that other cracks may show up before long. To nail the problem permanently, it would probably really be best to strip the gelcoat off & do the job properly.
Kind Regards,
Peter D.