I try to avoid much sanding during the framing stage. I may use 80 or 100 grit paper to rough sand the tip of the fin, the lower fin block or to shape the fin fillet or front turtle deck strip planking. Once the airframe is complete and your ready to start the finishing process Start with 150 grit and the block sander roughly 3-4 inches wide and 6-7 in length. All your doing here is basic leveling, do not get carried away here with just go over the entire airframe with the flat block - I do the curved areas of the turtledeck with the flat block, just make multiple passes over the curved areas. You want to pa attention to all the butt seams in the wood construction, the tabs and notches, all the intersections. try your best to level and feather as best you can.

At this point I apply the filler, you can use whatever you like, Mike turned me onto a product call "Super Fill" 2-part epoxy based that sands like balsa. I use that to fill in the fin fillet and all other areas of the plane that need it - it's a 12 hour cure so hit the entire plane. Once the filler is cured hit the filled areas with the 150 on the block and then move onto a 220 grit again covering the entire airframe.