Have you tried making a paste from Talcum Powder and Future?????
I tried this, and it worked really well.
Easy to sand, yes it dries whitish, but you cant have it all.
Mostly I use Tamiya basic putty.
To avoid the grainy texture of the stuff, after the final sand I go over it with some 1200 grade paper, and buff it up to a silky smooth finish.
After the buffing, it will not show up under light paints, except whites.