If you are in the US or know a place who sells it you can use a car body filler called Bondo. Some versions you need to mix the stuff but others can come pre-mixed. Some people also use wooder filler and plasterboard filler. I used a mix of super glue, wood filler and body filler for my helmets. All have there own advantages. To help with really big gaps or small missing bits you can mix baby powder in with the superglue
Primer/filler spray paint is pretty useful. personal I use a combo of primer/filler and Bondo. I've also heard mixed responses to XTC-3D, which is a printable coating for 3d prints that can then be sanded once it hardens, but it's also a bit expensive.
What I do is coat the entire inside/backside of my 3D print in Resin. I use smooth cast 65D. Then I coat my entire piece in XTC-3D. After a layer or two I prime it with filler primer and this removes the bulk of my print lines. I may do a small layer of bondo for larger gaps or seams.