For smoothing. I follow the same method as
Off Earth . Go over it with a low grit 60-80 sand paper for the whole thing or files for the hard to reach places. after that, move up to 120 to smooth it a little. 220-320 if you feel necessary.
If you want a REAL smooth finish, You will want to go over it with bondo spot putty. then sand that down with 220-320 grit (the spot putty is very easy to sand and fills and small divits and makes it very clean looking.
after that spray it with filler primer (2 in 1 rust-oleum makes). and then painting I don't have enough experience to help you with that. I just use spray paint for base coats. other people use air brushes.
see these pictures for how it ended up for my laser rifle
