CountCunning, thank you for you comments and you question. gmrhodes13 suggestion is correct. If I recall back to 2018 when I did this build, all this was fairly new to me. As you may have found from my post as well as others, Jimms fiberglass material does have a lot of small voids. Pinholes that are very hard to locate and fill. For the les flexible parts such as the helmet, I found a product at home depot called DAP Plastic Wood X. I mix some of this stuff with water and spread it on the helmet by hand. Once dry, you can start sanding, and this should fill most of those pinholes. Its also great on 3d printed stuff.
As for sanding, I start with 100 or 150 grit hand sanding and work through grits t 320 or 400. So the steps should be 100, 150, 180, 220, 320. for 400 and above, wet sand. It is not necessary to go all the way to 400, in fact 320 is probably the best grit to stop at and still ensure good adhesion for primer and paint. Pay close attention to the detail areas around the eyes, and panel lines. You can also use the DAP to enhance the eye bump if you desire.
Good luck on your build.