although you've been shunted to the sci-fi forum, there's one called 'scratch building', which the same techniques apply.
I use an auto body filler. it's a two-part 50-1 putty to catalyst. once set, it's sandable, drillable.
you could also try a one-part filler, like squadron green and white.
for making body parts, like hulls, i use flat styrene sheets from evergreen.