AWESOME!!

In all my multi-piece prints, I always do all the Gluing first.

I glue them all together, then I fill all the holes with whatever I can find... Wood putty, plastic putty... basically anything that can be sanded easily - I find just about all of them stick to plastic fairly well.

Once that's done then I sand it all off, get it super smooth... fix any holes I missed and then finally get it ready to primer spray.

I use a black or grey Matte undercoat (acrylic) then work on top of that. Since I dry brush nearly all my models by hand, I do alot of layers but for a storm trooper helmet, a nice white gloss and black gloss spray would do nicely.