look at this as a process. I've done a few pieces that were 3d printed and you'll be doing priming, sanding, re-priming, re-sanding until you have a completely smooth starting surface.
I did use some Bondo finish putty (~$3 in a tube at Wally World), but used sparingly. After applications of your primer, you'll be doing topcoats followed by wet sanding. Light and slow does it!