I ran a glider repair station for 20 years and have ground out many a crack.............never found any of them that extended into the underlining structure. I’m talking about cracks like those coming from spoiler box, a zig-zag looking crack is most likely indicating underlying structure has moved and must be ground out so as to see the structure below. You can get other opinions, in fact the Australian FAA, published an AD saying all cracks are structural and make the bird unairworthy!
Let the games begin............
JJ