Re: Hairline crack
if i am seeing correctly, the only crack i see in the pic is above the rub rail. Is that correct ? if so that means the crack is on the cap of the boat .
like YD said there would not be a way to fix it without messing up the gel coat.
You could look at some marine grade epoxy putty, but would have to sand around the crack for best adhesion and also for better transition acros the repair.
stay tuned i am sure you will get some great help here.