I remember how disappointed I was with the balsa filling process. Back then all we had was sanding sealer or balsa filler coat.
With the model glued together there was no way to get sandpaper to the four fin and four ribs root areas. I also had glued the rings in place.
It was probably then I started to fill balsa before building the model especially if there were lots of (impossible to reach) nooks and crannies.
If you fill the balsa and body tube spirals before building you should remove the primer spray (down to bare wood or bare body tube) at the fin and lug gluing areas. Otherwise your model might re-kit itself during boost or pop off fins at recovery.
Check out the recommended engines - a 1/2A6-2 in this one?