Can stress cracks be repaired so that they do not return or are they always going to return?

the best way is to use a dremel tool and grind the crack down to the fiberglass after that fill in the crack with gelcoat. After that wet sand the gelcoat to a smooth surface.

Will the crack return?

Yes,,,if you do it only with gelcoat it will.
You need to lay glass back in it so it bonds to the sides .

Grind it out to the galss and a little wider then the crack ,then lay some fiberglass in there,sand it down and match existing gelcoat. Buff it out ,,,,done.

But they still return ,just not as quick,,,,,there is always a reason for stresscracks so the problem is not the glass on that point its somewhere else. Could be a tight bulkhead or so.

I had good results that way though,4-6 years +.

Doing a small Apache right now ,,, always making the costumer happy .

I agree with what daredevil said. I would use MAS epoxy instead of poly resin. You can gelcoat over mas and epoxy is a little less brittle than polyester resin less likley to crack.

big fan of MAS epoxy. i have been doing jobs with that stuff for years with great results.

The cracks are on a 31 Sonic 1990. One is along the walkway on the starboard side a few feet infront of the driver. It is right there in the corner where the walkway ends and then the deck rises for the cabin, the other is in the cabin in the liner on the port side on the floor in the corner where the floor rises for the seat base. Both of these are around 1 1/2 feet long. Would you worry about this? The owner has had the boat for 4 years hasn't used it in 2 and says they have always been there since they had it. What do you think, worry or not?