When I build the Trajector before, I kind of messed up the fins. Which, given how basic this rocket is, kind of surprised me.
Each fin comes in two halves, like a clam. The instructions say to assemble the two halves with CA. Although that is brittle glue and not good for structural places, in this case it seemed to be OK. One end of the fin is mechanically held together by the motor mount, and the other end is mechanically held together by the pod.
On the prior builds, my mistake was more about dexterity. While I was holding the fins to apply glue, and while I was squeezing the two halves together, I got glue on my fingers. Which, as per the second law of thermodynamics, immediately transferred from my fingers to the surface of the fins. So, I goobered the appearance of the fins. The more I assembled, the more of a mess I made.
This time, I skipped the CA, and violated the rules by going with 15 minute epoxy to assemble the fin halves. I first scuffed up the inside of the fins with 180 grit sandpaper, to give the glue purchase. In fact, the smell of the plastic of this step was identical to the smell of cutting ABS plumbing pipes, so I think that's what was used.
To avoid gumming up the surface of the fins, I masked them off with blue tape. Now, if any glue squeezed out of the joint and onto my fingers and back onto the fin, its destructive influece would be blocked by the tape.
Once the fins dried, I did a similar technique for the pods, by masking them off to glue the halves together, and then masking off the fins before attaching the pods to the fins.