A while back I had mixed up a batch of epoxy thickened with microballoons (tiny glass spheres) and filled low spots in the centerboard and rudder. This lightweight fairing mixture sands very easily, and can be worked by hand or with the random orbit sander.
I used some partially worn out sandpaper and a flat block to smooth some areas, and backed up the sandpaper with the large wood cylinder for some of the curves where the blades transition to the flat areas.
I found I had a few spots that needed more filling, so marked those with pencil and applied more fill there.
I found that it's easier to tell where the curve is not fair by running my hand along the surface than it is to try to spot those areas by eye.