The above is what I do - I go to Lowes or Home Depot, pick up a level in tools, go to lumber and find the straightest, flat piece of 1 x 4 or 2 x 2 hard Oak or Poplar I can find according to the level, for cutting a sanding board out of. I put the level back on the shelf in tools. Cut the board to the length you feel comfortable with, attach adhesive back sandpaper from a roll to it and you can sand anything flat. For something like that area of a Belvedere fender, I would skim coat the area, applying it from top to bottom. Sand with the board and 80 grit moving at a 45 deg angle from bottom to top and alternate and sand at 45 deg in the opposite direction. That will keep you straight fore and aft but allow you to build in the vertical curve and match adjacent surfaces. A light dust with guide coat should show up any low areas that need another skim coat of filler. But you do have to be careful to avoid going to thick with the filler.