After flip flopping between possible finishing techniques, I settled on smoothing the print with XTC-3D resin and aluminum powder (Electroplating literally only lost because the stuff I chose to start with was out of stock. But electroplating
something is in my future. It looks exciting.)
This is the helmet right now: 1 coat of resin + aluminum powder, 1 round of hand sanding with 120 grit sandpaper (...I'm going to have a single buff arm...)
Its actually pretty smooth. You're seeing print lines but... also not. The lines are all filled, but the first round of sanding didn't hit all the low points, so I'll need to go over it again. And maybe a third time before I'm happy. Then I'll have at least one more coat of resin + aluminum before I start sanding to polish.
The resin mix has 2 bottles and the ratio of A:B is 2:1. When adding the aluminum (or other metal) powder, it's a 2:1:1 ratio. A:B:Powder
The black acrylic is a must. The resin is clear. The metal powder doesn't dissolve and won't hide the print lines. Tinting the resin will hide the print lines. (I forgot that step for 2 layers on a holocron I'm also working on and I can still see the danged print lines through the resin. And I didn't use quite enough for the 1st layer of the helmet).
Self Leveling: The XTC-3D resin is self leveling and amazing, so long as what you're "painting" it on is flat on the table. The helmet came pre-assembled, so it wasn't really an option.
The top of the dome was like glass and beautiful, but the rest of the print was bumpy because gravity was allowed to pull on the resin before it set. This is because I just made sure everything was covered and then left it to cure.
Further research shows that, after covering the helmet, I should have kept brushing the resin into place until it started to set. It probably wouldn't have needed such aggressive sanding. I still have 1 more coat (at least), so I'll do it right for that one.