If a crack repair is not properly done they will show up after a paint
job. The quick fix done by auto repair shops of slappng Bondo on such a large crack will never be adequate for as deep of a crack as your photo is showing. What you are seeing is what will always happen with the wrong approach to the repair.
West Systems Epoxy company has produced a pdf document with the instructions in it for fixing your crack. http://www.westsystem.com/ss/assets/...aintenance.pdf
Go to chapter 2. Be sure to look at the illustration where they show modifying an inexpensive can opener to slightly widen the crack and then pay attention what they say to use for filling the crack. You don't have to use that brand of epoxy but you should not try to use Bondo filler for the job, not even the type with fiberglass in it as it does not have as much strength as epoxy does. Use epoxy and follow the instructions in the manual.
Do not just slap some Captain
Tolley's in that crack, you need to do the job the right way. Captain
Tolley's is a very good product but it is not good enough for this situation. It is meant for filling hairline cracks, it is not for structural repair. If you put it in the crack now that means it is going to be in the way of the epoxy bonding to the fiberglass shell and that means your repair will be substantially weaker than doing it the right way.
Do note that crack might actually go all the way through the shell and if it does you should be putting a layer of fiberglass cloth over the area from inside of the shell so that the area is properly reinforced. A long crack in a complex area that goes through the shell is indicating that area is very much in need of extra strength. In that case a filler that only goes on the thin edges of a crack is not a sufficient repair as there will still be too much stress on the thin shell and the crack will reopen once again. What will be needed for that fix is those extra layers of fiberglass on the inside of the shell so that the thickness is built up to be sufficiently strong for the stress placed on it.