More about vertical versus horizontal grain. The two photos below show the difference. In wood like this, the darker parts are harder than the lighter parts. With horizontal grain, the dark and light parts are very tightly packed together. If you can imagine, when you are sanding it, there is not enough space between the harder dark parts for the sand paper “drop” into, thus the surface will remain flat and smooth. But when you have horizontal grain, the dark parts are separated by large expanses of soft wood that the orbital sander can “drop” into, thus resulting in an uneven surface. In comes the fairing board to save the day – its extra length prevents it from “dropping” into the softer parts of the grain.
I tried to avoid horizontal grain and I thought I had cut all my strips correctly, but one of my boards apparently changed grain over its length. Those two photos show the same six strips, one photo at one end of the kayak and one at the other. Clearly the grain has changed over the length of the strips. Perhaps that log was bent or twisted to somehow cause this abnormality. Anyway, I could not have avoided having at least some horizontal grain from that board.