Firstly sand the fus smooth as you can
I then use Dope and mix it with talcum powder to the consistencey I require (fairly thin) Apply this with a brush or paint scraper.
Allow this to dry and sand as normal
This will fill grain in the wood and adds next to no weight to the aircraft
Only problem is it is fairly dusty when sanding.
I also use this method at a thicker consistency to fill minor dents etc
Undercoat
Paint
I use automotive acrylics