I've already got an ARTF trainer and a secondhand plane and am now building my first plane (WOT4 Mk 3). The wings are veneer covered foam and the fuselage is a balsa sheet construction (no areas of open frame). I'm looking for advice on how to achieve a really smooth sanded finish to all that bare wood before covering with Solarfilm (similar to monokote). Any special recommendations on filler, etc. What sandpaper or wet/dry grit nos. I'm very patient and will go through all the stages necessary to achieve a really good looking finish.

Rough sand with, say 150 grit. Slop lots of sanding sealer on it. Let dry and sand with 220.
For filler, I use Elmer's wood filler at Home Depot. Sometimes dents can be removed by wetting, with water or alcohol, then iron them out. Be careful if you use alcohol, as the flash temp is very low.