The plans for the kayak suggest using a router with a 3/8" round down bit to make a nice, curved edge on the inside of the coaming. The router is an intriguing tool, primarily because I haven't got one. You can do all kinds of neat things with a router, but they aren't the neat kind of things that I commonly do. Still... an opportunity for a new power tool... well, it's nasty and rainy out, so I'll pass.
Instead, I grabbed one of the three rasps that came in the $2.49 set that I bought at HF to cut a notch in the side of the hull by the hatch covers. The notch is a half-circle that lets me get a finger under the edge of the flush hatch covers to remove them. I needed a half round rasp, but using half of the full round rasp that came in the set was mathematically equivalent. For the coaming edge, I used the flat rasp. It makes a very rough cut and a very annoying sound (one could almost describe it as 'raspy,' if one was willing to make such a gawdawful pun), but it cut away the wood more quickly than the trip to HF would have taken.
Going back over the edge with 100 grit sandpaper smoothed it down nicely. Now I have to cover it up with fiberglass cloth and epoxy.