Do you guys prime before filling so the putty will stick better or do you putty first and then prime?and what primer do you use?

I putty/ fill, sand, then prime with Tamiya white fine primer. I use either Squadron or Tamiya putty. The putty adheres well to the plastic.

^^Ditto^^. I actually never prime, but that is another story. The putty (like glue) actually sticks better to bare plastic. Most putties have some type of solvent in them so they chemically bond w/the plastic. Paint/primer can cause the putty to eventually come loose as it creates a barrier between the putty and plastic.

I guess I kinda do both. I fill the obvious stuff first before priming. I use Tamiya putty and Liquid Paper correction fluid. After I prime I may go back and touch up any problem spots.

this. I think of the first coat of paint as a "guide coat" to even out any materials' natural finish (brass/ kit styrene/ scratch-built white styrene/ resin/ wire/ first layer putty) and give me a fresh look at the project.

with a less obstructed view, "fresh eyes", I make any course corrections and then usually only prime those areas to blend.

after prime, undercoat (which is sometimes the first coat depending on colors), and two top coats since I still am a bit trepidatious spraying a heavy coat without sags.