Welcome to the 405th CrazyViking89 ! I'd say go over the hard-surface parts of the model to try and remove a few of the bumps that are on/in the corners (in these areas for example):
You should be able to take away most of the unwanted material away with some needle files, and then some sandpaper (I'd say roughly 300 grit) to clean up afterwards. It looks like you've covered the helmet in a layer of paint, do you know if it's sandable?
For the skull itself, I wouldn't stress too much on making it smooth. As long as there aren't any layer lines or seams, any imperfections in the surface will actually add texture to the skull making it more realistic. That's what I love about organic shapes!
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