First, I sanded with 60 grit paper on my random orbital sander, then 100 grit aluminum oxide paper on my pad sander. I've mentioned this to other builders, and they haven't noticed the mottled effect. At the companionway opening, I cut the opening in the roof section too wide. It is a long, thin, triangular section that would be tedious to fill with wood, and would be fragile as well. Here, I'm using pvc tape coated plywood clamped under the area and another piece to define the edge.
Tape also goes on the c-clamp to keep it from being permanently affixed to the boat.
The c-clamps, and pvc tape coated wood, snaps away clean, leaving a filler piece that is strong and permanent.