Two boo-boos.
The first is just plain old sand through the color -- oops. Clearly I didn't have as much clear there as I thought. That'll get spotted in, and there'll be, well, a spot because I didn't do a good job with the original color coat. Next time...
The second one is more interesting. The fins are made of 1/4" balsa, with a rectangular cut-out for the stab to go into. This means, inevitably, that there's a huge discontinuity in the wood right there -- which is going to show no matter what I do. Even if I had gone through more iterations of clear coat and sanding (or did so now), it'd be smooth at the temperature I sanded it at, but show a ripple at other temperatures (like, in the sun at a contest). So, next time I do this I'll laminate the part -- for something like this I'll use 3/16" and 1/16" wood, and just make the cutouts in the 3/16" part. There will still be a discontinuity, but hopefully it won't show.
Being me, this probably means I'll make two left fins, but that's my own personal cross to bear.