I  spray  the  assembled  cars  with  gray  primer  on  the  outside,  and  tan  on  the  inside,  and very  lightly  sand  the  outside  before  either  spray  or  brush  painting  them.  sometimes,  when  I  feel  organized,  I  spray  sub  assemblies,  but  it  always  needs  some  more  on  the  outside  when  fully  assembled.