I know everyone's looking for lightweight fillers, but I am a boat builder and the amount of filler we need sometimes does not need to be lightweight more than it needs to be visually correct when filling outside joints that will stay clear-coated.

Instead of commercial epoxy fillers (=expensive) or saw dust (=coarse) I use wheat flour. You know, the stuff for bread dough.

It is:

1. Close to wood as a material.
2. Much finer than wood dust from sawing or sanding, so creates smoother surfaces.
3. Almost wood color (light).
4. Available everywhere.
5. Very cheap.
6. Ecological. When You need to get rid of the excess, just bake a bread and eat it.

Further hint:

If You need to match filler color to wood color, try using other types of flour also: rye, barley, corn...
And try coloring it with ground cinnamon, ginger...