for resin, slow setting CA mixed with either baking soda or talc powder. fills pin holes and
other surface irregularties and won't shrink as much. sands easily in 15 min's.
you could also use 5 min. epoxy, liquid steel, spackle, auto body filler - all depends on what you got on hand at the time.
3d modelling is an option
a true mental excercise in frusrtation